Enhanced Electrocatalytic Activity of Stainless Steel Substrate by Nickel Sulfides for Efficient Hydrogen Evolution

Round 1

Reviewer 1 Report

Overall comment:

The introduction part needs to be modified to tell the story in a better way. The novelty of the research is not mentioned correctly and lacks the motivation of the current study. The manuscript needs to be checked by a native English speaker.

I am suggesting minor revisions to the manuscript.

Specific comments:

Abstract:

• Line 20 pg 1: What do you mean by 'easy methods'? Please use better wording.

Introduction:

• Line 30 pg 1: Can it really substitute fossil fuel now? Don't think electrochemical water splitting is at such a stage that it can replace even 10% of fossil fuel. You need to change the approach to write the background. Mention at what stage the electrochemical water splitting technology is now.
• Line 32 pg 1: Mention why Pt is so effective for HER. What characteristics of Pt makes it so effective? What is missing in other metals to act as an effective catalyst? What are the possible alternatives to Pt? You need to include all of this in the literature review.
• Line 42 pg 1: why the synthesis of NiSx is challenging?
• I believe you can improve your literature review by systematically presenting the story. Not enough literature review was performed for choosing Nickel sulfide and the method to prepare it. Also, the novelty was not stated correctly.

Results:

• Line 72 pg 2: “This synthetic approach is very facile and can be easily to scale up” Can you please elaborate on this.
• Line 83 pg 3, Line 90 pg 3: replace “previous researches” with previous studies
• Line 161 pg 6: Figure 6 is too crowded. You may want to split it into two figures.

Conclusions:

• Line 194 pg 7: “difficulty to synthesize nickel sulfides through the electrodeposition and sulfurization” this point should be mentioned in the introduction to build the novelty of the work.

Author Response

Reviewer 1

Reviewer 1’s general comments: The introduction part needs to be modified to tell the story in a better way. The novelty of the research is not mentioned correctly and lacks the motivation of the current study. The manuscript needs to be checked by a native English speaker.

I am suggesting minor revisions to the manuscript.

Response: The reviewer raises a good point. We have modified the abstract (given in the response for the reviewer 1’s comment 1), and some parts of the introduction (given in the response for the reviewer 1’s comment 2, 3, and 4).

We checked this manuscript by a native English speaker and attached the certificate of editing below:

Reviewer 1’s Comment 1: Line 20 pg 1: What do you mean by 'easy methods'? Please use better wording.

Response: We are thankful for the reviewer’s constructive comment. As suggested, we have changed the aforementioned point as follows (Line 19-10)

“Herein, we successfully developed HER catalyst composed of NiS_x (x = 1 or 2) on stainless steel (NiS_x/SUS) using electrodeposition and sulfurization techniques.”

Also, the abstract was edited as follows (Line 15-23):

“Water splitting is one of the efficient ways to produce hydrogen with zero carbon dioxide emission. Thus far, Pt have been regarded as a highly reactive catalyst toward hydrogen evolution reaction (HER); however, the high cost and rarity of Pt significantly hinder its commercial use. Herein, we successfully developed HER catalyst composed of NiS_x (x = 1 or 2) on stainless steel (NiS_x/SUS) using electrodeposition and sulfurization techniques. Notably, the ECSA (electrochemical active surface area) of NiS_x/SUS was improved more than two order of magnitudes, resulting in considerable improvement in the electrochemical charge transfer and HER activity in comparison with bare SUS. The long-term HER examination by linear scan voltammetry (LSV) confirmed that NiS_x/SUS was stable up to 2000 cycles.”

Reviewer 1’s Comment 2: Line 30 pg 1: Can it really substitute fossil fuel now? Don't think electrochemical water splitting is at such a stage that it can replace even 10% of fossil fuel. You need to change the approach to write the background. Mention at what stage the electrochemical water splitting technology is now.

Response: This is an important point. We totally agree with the reviewer’s opinion that fossil fuels (e.g., petroleum, natural gas, and coal) have been providing a major proportion of all the energy being consumed globally and the current technologies cannot completely replace the whole infracstructure well-established for fossil fuel. However, the excessive dependence on such non-renewable energy sources has become a critical global challenge, including the exhaustion of fossil fuel reserves, carbon dioxide emission, and air pollution. Therefore, seeking for alternative sustainable energy resources to replace fossil fuel is considered very important to resolve the energy crisis in a long-term plan. Among those, hydrogen is regarded as an ideal energy carrier that can potentially substitute conventional fossil fuel because of its large energy density and zero carbon dioxide emission. It should be noted that this topic, particularly hydrogen evolution reaction (HER) via water electrolysis, has received tremendous attention as evidenced by numerous research and review papers addressing scientific and technical issues related to hydrogen production [1-2].

However, due to the limitation in the manuscript length, we only selected some of the keynotes and representative references with respect to the background of HER into the manuscript. For more information, we would encourage readers to refer to the cited references.

1 Roger, I.; Shipman, M.A.; Symes, M.D. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting. Nat. Rev. Chem. 2017, 1, doi:10.1038/s41570-016-0003.

2 Hisatomi T.; Domen K. Reaction systems for solar hydrogen production via water splitting with particulate semiconductor photocatalysts. Nature catalysis. 2019, 2, 387-399, doi:10.1038/s41929-019-0242-6.

Reviewer 1’s Comment 3: Line 32 pg 1: Mention why Pt is so effective for HER. What characteristics of Pt makes it so effective? What is missing in other metals to act as an effective catalyst? What are the possible alternatives to Pt? You need to include all of this in the literature review.

Response: This is another critical theory background. Theoretically, it is found that the Gibbs free energy for hydrogen adsorption (∆G_H*) of Pt is close to zero, which means Pt has the optimal binding energy with adsorbed atomic hydrogen during the HER process. This renders Pt highly reactive toward HER. Beside Pt or noble metal, metal chalcogenide, nitrides, phosphide could be alternative candidates to replace Pt. For more information regarding the HER mechanism, readers can refer to the following references [1]

1 Roger, I.; Shipman, M.A.; Symes, M.D. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting. Nat. Rev. Chem. 2017, 1, doi:10.1038/s41570-016-0003.

Reviewer 1’s Comment 4: Line 42 pg 1: why the synthesis of NiSx is challenging? I believe you can improve your literature review by systematically presenting the story. Not enough literature review was performed for choosing Nickel sulfide and the method to prepare it. Also, the novelty was not stated correctly.

Response: It is important to clarify this statements. Actually, for the fabrication of NiS_x, there have been a number of synthetic protocols (e.g., hydrothermal, sol-gel) but the two-step synthetic approach presented in this study has been rarely reported. The electrodeposition of nickel, followed by sulfurization is critical to enable effective and stable growth of NiS_x directly on SUS. This is important technical issue to prevent undesired detachment of catalysts from the SUS surface during HER process.

We also modified a part in the introduction as follows (Line 40-43):

“Although a number of synthetic methods for nickel sulfide (e.g. solvothermal and hydrothermal [7,9], and electrospinning [12] have been reported, it is still very important to establish an effective and simple approach to fabricate the nickel sulfide- based electrodes for HER application.”

Reviewer 1’s Comment 5: Line 72 pg 2: “This synthetic approach is very facile and can be easily to scale up” Can you please elaborate on this.

Response: In this study, electrodeposition technique for introducing nickel overlay on the surface of SUS. After that, the Ni-deposited SUS was annealed in sulfur environment at 300℃. These steps are quite straightforward and do not required very complicated instrument. Therefore, it could be easily to scale up. Detailed NiS_x/SUS synthesis method is provided in Section 2.1.

Reviewer 1’s Comment 6: Line 83 pg 3, Line 90 pg 3: replace “previous researches” with previous studies

Response: We appreciate this reviewer’s concern and we now replaced “previous researches” with “previous studies” in Line 85 and 92.

Reviewer 1’s Comment 7: Line 161 pg 6: Figure 6 is too crowded. You may want to split it into two figures.

Response: We are thankful for the reviewer’s constructive comment. Indeed, we would like to show both HER performance and the HER mechanism combined in one figure which could help readers to understand our point. In this case, I think it is more appropriate to keep this figure as it is.

Reviewer 1’s Comment 8: Line 194 pg 7: “difficulty to synthesize nickel sulfides through the electrodeposition and sulfurization” this point should be mentioned in the introduction to build the novelty of the work.

Response: We really appreciate the reviewer’s suggestion. As suggested, we have added this point into the introduction and conclusion.

Change in the introduction (Line 58-62):

“The following sulfurization at relatively low temperature, whereas preventing the metal diffusion from SUS to the surface catalytic phase, is sufficient for the direct growth of NiS_x on the SUS. The reported synthetic approach is important to prevent undesired detachment of NiS_x from the SUS surface during HER process”

Change in the conclusion (Line 199): 

“In this research, we overcame the limitations of SUS as a substrate and directly synthesized nickel sulfides through the electrodeposition and sulfurization”

Sincerely yours,

Prof. Ki-Joon Jeon

Author Response File: Author Response.pdf

Reviewer 2 Report

In this manuscript, the authors reported a simple synthesis of  hybrids of NiS and NiS2 on stainless steel substrate to achieve the limitations of SUS and difficulty synthesizing nickel sulfides for an efficient HER electrode. As-prepared nickel sulfides on SUS show that ECSA (electrochemical active surface area) of nickel sulfide on SUS was improved more than 140 times compared with SUS and Rct (charge transfer resistance) was considerably decreased compared to SUS. I agree that the proposed synthesis methods by electrodeposition and sulfurization as well as HER performance are attractive. Therefore, I recommend the current manuscript for publication in the journal of Catalyst after the following points are addressed:  

• The elemental content of materials should be given in the table. Especially, the sulfur amount on SUS will be discussed
• The thickness of NiS and NiS2 on stainless steel substrate
• How the stability of NiS and NiS2 on stainless steel change after HER cycles?
• XPS and XRD data results should be discussed in depth. XPS and XRD results can be compared with the following papers. A) Palapati, N. K. R., et al. "Enhancing the electronic conductivity of Lignin-sourced, sub-micron carbon particles." 2015 IEEE Nanotechnology Materials and Devices Conference (NMDC). IEEE, 2015.
• The introduction part should be re-written to upgrade the importance of this work, and the discussion part should be doing more explanation for the discussion in depth. But I found there is more description of the experimental phenomenon with little discussion for the possible reasons.
• The writing style, grammar and language usage should be checked by native speakers.

Author Response

Reviewer 2

Reviewer 2’s general comments: In this manuscript, the authors reported a simple synthesis of hybrids of NiS and NiS2 on stainless steel substrate to achieve the limitations of SUS and difficulty synthesizing nickel sulfides for an efficient HER electrode. As-prepared nickel sulfides on SUS show that ECSA (electrochemical active surface area) of nickel sulfide on SUS was improved more than 140 times compared with SUS and Rct (charge transfer resistance) was considerably decreased compared to SUS. I agree that the proposed synthesis methods by electrodeposition and sulfurization as well as HER performance are attractive. Therefore, I recommend the current manuscript for publication in the journal of Catalyst after the following points are addressed.

Reviewer 2’s Comment 1: The elemental content of materials should be given in the table. Especially, the sulfur amount on SUS will be discussed

Response: This is a very helpful comment. As suggested, the elemental content of materials for NiS_x/SUS and S-SUS was estimated by XPS and tabulated (table below). We have included the results in the revised version and supplementary material.

Table S1. Surface chemical analysis of samples by XPS.

Reviewer 2’s Comment 2: The thickness of NiS and NiS2 on stainless steel substrate.

Response: This is another important point. For this, scanning electron microscope (SEM) was used to to determine the thickness of NiS_x on SUS. The data shows that the thickness of NiS_x on SUS substrate is about 24 µm. This data and related comment were added into the manuscript (Line 84-85) and supporting information (Figure S1).

Fig. S1. The thickness of the SEM image of NiS_x/SUS.

Reviewer 2’s Comment 3: How the stability of NiS and NiS2 on stainless steel change after HER cycles?

Response: This is an important point. To confim the stability, NiS_x/SUS was subjected to 2,000 cycles of LSV in 1M KOH. After that, the sample was analyzed using XPS. As seen, after 2,000 cycles the characteristic XPS peaks of Ni 2p in NiS_x. This result was added into the manuscript. The line number are Line 155-159:

“In addition to this, figure S4 shown XPS analysis of NiS_x/SUS after 2,000 cycles of LSV, suggesting that NiS_x still stable after the HER activity in alkaline condition.”

Fig.S4. X-ray photoelectron spectroscopy (XPS) of NiS_x/SUS after 2,000 cycles in 1M KOH. (a) Ni 2p and (b) S 2p.

Reviewer 2’s Comment 4:XPS and XRD data results should be discussed in depth. XPS and XRD results can be compared with the following papers. A) Palapati, N. K. R., et al. "Enhancing the electronic conductivity of Lignin-sourced, sub-micron carbon particles." 2015 IEEE Nanotechnology Materials and Devices Conference (NMDC). IEEE, 2015. B) Altinci, OC Cem" Energy & Fuels 2020, 34, 6, 7658–7665 https://doi.org/10.1021/acs.energyfuels.0c01103. C) http://doi.org/10.1016/j.matchemphys.2018.06.008

Response: We are thankful for the reviewer’s constructive comment. As suggested, we now edited the manuscript and added references (#12, 25, and 26) in Line 89- 98 and 105-107:

“The crystallinity of the prepared electrodes were investigated using X-ray diffraction (XRD) analysis performed in the 2θ range from 20 to 90°. Fig. 3 shows the XRD results of SUS, S-SUS and NiS_x/SUS. SUS shows normal FCC (Face centered cubic) crystal structure of (111), (200) and (220) at 43.5, 50.7 and 74.5° respectively, indicating consistency with previous researches [21], and there is no significant difference from XRD results between SUS and S-SUS, suggesting that SUS is relatively passive toward sulfur vapor at low temperatures. After the sulfurization of nickel on SUS, the evolution of new set of diffraction peaks can be assigned to NiS ((100), (101), (102), and (110) at 30.1, 34.6, 45.7, and 53.5°, respectively)[7] and NiS₂ ((200), (210), (211), (220), and (311) at 31.4, 35.3, 38.8, 45, and 53.4°, respectively)[13]. The peak intensity corresponding to those characteristic of SUS is seen to lessen to some extent, implying that the surface of SUS was covered with a thick layer of NiS_x.”

“In order to investigate the chemical composition and binding states of the prepared electrodes, XPS analysis was carried out as shown in Fig 4. The XPS survey spectra of SUS, which mainly consists of Fe, Ni, Cr and Mn, shows high intensity of O1s and C1s and very low intensity of Fe 2p indicating metal oxide film on SUS (Fig. 4a) [22]. In case of S-SUS, because the intensity of Fe 2p and Cr 2p was too small to quantify the metal sulfides, S 2p was analyzed by four peaks at 163.98 eV, 163.08 eV, 162.3 eV, and 161.2 eV (S₀, S_n^2-, S₂^2- and S^2- ) indicating the metals-S bonds (Fig. S2) [23–25].”

Reviewer 2’s Comment 5: The introduction part should be re-written to upgrade the importance of this work, and the discussion part should be doing more explanation for the discussion in depth. But I found there is more description of the experimental phenomenon with little discussion for the possible reasons.

Response: We are thankful for the reviewer’s constructive comment. As suggested, we have changed the abstract and introduction. The abstract was edited as follows (Line 15-23):

“Water splitting is one of the efficient ways to produce hydrogen with zero carbon dioxide emission. Thus far, Pt have been regarded as a highly reactive catalyst toward hydrogen evolution reaction (HER); however, the high cost and rarity of Pt significantly hinder its commercial use. Herein, we successfully developed HER catalyst composed of NiS_x (x = 1 or 2) on stainless steel (NiS_x/SUS) using electrodeposition and sulfurization techniques. Notably, the ECSA (electrochemical active surface area) of NiS_x/SUS was improved more than two order of magnitudes, resulting in considerable improvement in the electrochemical charge transfer and HER activity in comparison with bare SUS. The long-term HER examination by linear scan voltammetry (LSV) confirmed that NiS_x/SUS was stable up to 2,000 cycles.”

The introduction was also modified in part to highlight the novelties of our work.

Reviewer 2’s Comment 6:The writing style, grammar and language usage should be checked by native speakers.

Response: We checked this manuscript by a native English speaker and attached the certificate of editing below:

Sincerely yours,

Prof. Ki-Joon Jeon

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments to the Authors:

The manuscript by Youn et al. demonstrated the synthesis of nickel sulfide on the SUS grade stainless steel substrate via electrochemical deposition of Ni species followed by thermal sulfurization for hydrogen evolution reaction (HER). In terms for methodology and scientific merit, this manuscript reported some new data and might be beneficial for the researchers working in the similar research area. Therefore, this manuscript can be considered for publication after a “Major Revision”, subjected to the following comments:

1. Is it the first time that authors used this SUS grade steel as a substrate to grow catalyst for HER? Has any type of steel previously used as the substrate to grow electrocatalyst? Author must discuss such information in light of previously published articles at the introduction section.
2. Authors should take an attempt to calculate the surface elemental composition and present in the manuscript as the surface composition can significantly impact the electrocatalytic performance of a catalyst.
3. According to the XRD pattern, the formed nickel sulfide on the SUS surface is the mixture of NiS and NiS₂. Have authors tried to identify the active nickel sulfide component for HER? If so, which form of NiS_x contributed the most?
4. It is good to see a slight change of LSC curve after 2000 sweeps, meaning the good stability of this reported electrode system. Did the authors perform XPS and XRD measurement after this cycling test? It is important to know the surface or bulk chemical structure of the electrode after a certain electrocatalytic testing.
5. Was there any leaching of metallic species during/after electrocatalytic tests? Did the author analysed the electrolyte solution after electrocatalysis to check any metallic species, especially Ni and Cr?   

Comments for author File: Comments.pdf

Author Response

Reviewer 3

Reviewer 3’s general comments: The manuscript by Youn et al. demonstrated the synthesis of nickel sulfide on the SUS grade stainless steel substrate via electrochemical deposition of Ni species followed by thermal sulfurization for hydrogen evolution reaction (HER). In terms for methodology and scientific merit, this manuscript reported some new data and might be beneficial for the researchers working in the similar research area. Therefore, this manuscript can be considered for publication after a “Major Revision”, subjected to the following comments.

Reviewer 3’s Comment 1: Is it the first time that authors used this SUS grade steel as a substrate to grow catalyst for HER? Has any type of steel previously used as the substrate to grow electrocatalyst? Author must discuss such information in light of previously published articles at the introduction section.

Response: This is important to clarify. Indeed, SUS has been used for HER because SUS has mechanical strength, corrosion resistance, cost-effectiveness, and good electrical conductivity, so it is valuable as a substrate for HER. However, SUS itself is not active and it necessitate integrating with HER active components. Accordingly, many researchers are trying to synthesize various metals or two-dimensional materials on the surface of SUS. However, growing electrochemically active materials on the surface of SUS remains a challenging task. This is mainly because SUS consists of many kinds of metals such as iron, nickel, molybdenum, chromium and magnesium, and those metals can be inevitably diffused to surface at high pressure and temperature, and that prevents researchers in controlling the material composition and in understanding the composition-HER performance correlation. Further, the slippery surface of SUS and the generation of hydrogen bubbles during the HER cause the undesired detachment of catalysts from the SUS and the electrode eventually loses its reactivity. We have added references (#17-20) into the manuscript were revised.

Reviewer 3’s Comment 2: Authors should take an attempt to calculate the surface elemental composition and present in the manuscript as the surface composition can significantly impact the electrocatalytic performance of a catalyst.

Response: This is a very helpful comment. As suggested, the elemental content of materials for NiS_x/SUS and S-SUS was estimated by XPS and tabulated (table below). We have included the results in the revised version and supplementary material.

Table S1. Surface chemical analysis of samples by XPS.

Reviewer 3’s Comment 3: According to the XRD pattern, the formed nickel sulfide on the SUS surface is the mixture of NiS and NiS₂. Have authors tried to identify the active nickel sulfide component for HER? If so, which form of NiS_x contributed the most?

Response: This is really an interesting inquiry. Based on the XRD and XPS data, it was found that the NiSx was the mixture of NiS and NiS₂. Unfortunately, we failed to isolate each nickel sulfide form and therefore we could not identify the most active component for HER at the current stage.

Reviewer 3’s Comment 4: It is good to see a slight change of LSC curve after 2000 sweeps, meaning the good stability of this reported electrode system. Did the authors perform XPS and XRD measurement after this cycling test? It is important to know the surface or bulk chemical structure of the electrode after a certain electrocatalytic testing.

Response: This is an important point. To confim the stability, NiS_x/SUS was subjected to 2,000 cycles of LSV in 1M KOH. After that, the sample was analyzed using XPS. As seen, after 2,000 cycles the characteristic XPS peaks of Ni 2p in NiS_x. This result was added into the manuscript. The line number are Line 155-159:

“In addition to this, figure S4 shown XPS analysis of NiSx/SUS after 2,000 cycles of LSV, suggesting that NiS_x still stable after the HER activity in alkaline condition.”

Fig.S4. X-ray photoelectron spectroscopy (XPS) of NiS_x/SUS after 2,000 cycles in 1M KOH. (a) Ni 2p and (b) S 2p.

Reviewer 3’s Comment 5: Was there any leaching of metallic species during/after electrocatalytic tests? Did the author analysed the electrolyte solution after electrocatalysis to check any metallic species, especially Ni and Cr?   

Response: This is an important comment. To confirm the stability of NiS_x/SUS, we have used LSV (2,000 cycles) and XPS (details given in the reviewer 3’s comment 4) to analyze the chemical surface composition and elemental oxidation states. The data suggested that NiS_x/SUS could sustain for continuous HER examinations. Unfortunately, we could not obtain meaningful data regarding the leaching of metallic species during/after electrocatalytic tests such as Inductively coupled plasma – optical emission spectroscopy (ICP-OES).

Figure. Survey spectra of NiS_x/SUS after 2,000 cycles.

Sincerely yours,

Prof. Ki-Joon Jeon

Author Response File: Author Response.pdf

Reviewer 4 Report

Youn et al. describe a method to enhance the performance of stainless steel (SS) electrodes in HER, by covering the SS substrate with NiSx via electrodeposition and sulfurization. They provide comprehensive structural and morphological characterization of modified electrodes and electrochemical performance evaluation clearly showing superior performance of their modified material compared to bare SS. The paper is suitable for publication upon addressing the following comments:

• The figure showing XPS spectra should be named Figure 4 instead of Figure 3.
• Page 5 line 123, SCE should be defined
• The authors observed largely improved performance of NiSx/SUS compared not only to SUS but also S-SUS. Do the authors have an explanation for this result? Does this suggest that Ni is the most important component here? Can this be true for the SS substrate as well? The authors should add some comments in this regard.
• in general the language can be improved. To give some examples: i) page 5 line 130: "is involved in a" should be "involves a"; ii) page 6 line 149: "was the smallest" should be "had the smallest". I recommend the authors to read everything once again and correct similar errors.

Author Response

Reviewer 4

Reviewer 4’s general comments: Youn et al. describe a method to enhance the performance of stainless steel (SS) electrodes in HER, by covering the SS substrate with NiSx via electrodeposition and sulfurization. They provide comprehensive structural and morphological characterization of modified electrodes and electrochemical performance evaluation clearly showing superior performance of their modified material compared to bare SS. The paper is suitable for publication upon addressing the following comments.

Reviewer 4’s Comment 1: The figure showing XPS spectra should be named Figure 4 instead of Figure 3.

Response: The Figure number confirmed and changed of the whole manuscript has been improved.

Reviewer 4’s Comment 2: Page 5 line 123, SCE should be defined

Response: As suggested by the reviewer we have now included the unabbreviated name about SCE in the revised version of the manuscript. The line number are Line 126-127:

“NiS_x/SUS was used as a working electrode, Pt and Saturated calomel electrode (SCE) were used as a counter electrode and reference electrode respectively.”

Reviewer 4’s Comment 3: The authors observed largely improved performance of NiSx/SUS compared not only to SUS but also S-SUS. Do the authors have an explanation for this result? Does this suggest that Ni is the most important component here? Can this be true for the SS substrate as well? The authors should add some comments in this regard.

Response: The Nickel-based catalysts have known studied for hydrogen production as one of the promising non-noble metal alternatives because of satisfactory electrocatalytic activity and stability in alkaline conditions. Also, the HER activity of Nickel on SUS shown improved performance compared to the S-SUS. However, we confirmed that NiS_x/SUS is the most efficient catalyst for HER activity.

Figure. LSV in the range of 0 to -0.6 V vs RHE in 1M KOH.

Reviewer 4’s Comment 4: In general the language can be improved. To give some examples: i) page 5 line 130: "is involved in a" should be "involves a"; ii) page 6 line 149: "was the smallest" should be "had the smallest". I recommend the authors to read everything once again and correct similar errors.

Response: As suggested by the reviewer we have now changed the “had the smallest” in the revised version of the manuscript. The line number are Line 152:

“On the other hand, R_ct of NiS_x/SUS had the smallest value of 11.1 Ω,”

Also, we checked this manuscript by a native English speaker and attached the certificate of editing below:

Sincerely yours,

Prof. Ki-Joon Jeon

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The authors have satisfactorily addressed the queries and the manuscript should be accepted in its current form.