Green Synthesis of Flowerball-like MoS₂/VC Nanocomposite and Its Efficient Catalytic Performance for Oxygen Reduction Either in Alkaline or Acid Media

Round 1

Reviewer 1 Report

Dear Editor of Catalyst,

I have completely studied the article. The authors have prepared an article entitled “Green Synthesis of Flowerball-like MoS2/VC Nanocomposite and Its Efficient Catalytic Performance for Oxygen Reduction Either in Alkaline or Acid Media”. The article has been written well. I think the subject of Oxygen Reduction is important since many articles were published on this subject. The synthesized material was fully characterized. The article has been written well. The current version of this article can be accepted only after a minor revision.

1. The authors should provide a more strong background for Oxygen Reduction by different catalysts.

Please also cite some more updated articles and reviews on this subject.

2. Please compare the significance of this catalyst with the catalysts reported in the articles mentioned below:

https://www.sciencedirect.com/science/article/abs/pii/S0021979721016763

https://www.sciencedirect.com/science/article/abs/pii/S1385894721024621

3. I think the catalyst is so active for important reactions such as CO₂ capture, If possible check the catalyst for a CO₂ capture reaction as well.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Review on

“Green Synthesis of Flowerball-like MoS2/VC Nanocomposite and Its Efficient Catalytic Performance for Oxygen Reduction Either in Alkaline or Acid Media”

 

 

In this work, Zhang et al.  present a study about flowerball-like nanocomposites of the type MoS₂/Vulcan and its ORR activity in alkaline and acidic media. While the manuscript is well structured, there are too many figures especially with respect to the text and the discussion of the findings. Please reduce to max. 10 figures and try to highlight the main findings.

Please find my general and specific comments below.

 

• What is green about the synthesis? How is MoS₂ synthesized normally and what are the differences?
• Generally, the language has to be improved.
• In the abstract: It is difficult to estimate the duration given in second, please convert to hours or something more applicable.
• In the introduction: Please clearly distinguish between batteries and fuel cells.
• Experimental Part: Are the scanning range of the electrochemical treatment are given with respect to the RHE scale? If so, why for example cycling between -0.4 to 0.8 V?
• Generally, there is too few literature assessed for the comparison of activity and the discussion.

 

Line 57: Highly porous carbons are mentioned and a few lines below VC is mentioned as suitable candidate. Is VC really a highly porous carbon?

Line 81: Instead of naming the samples after the used amount of VC, maybe it is more applicable to determine the weight loading of MoS₂ on VC? Or does the VC act as more as a support?

Line 138: If the layers would be disordered, one would not see the diffraction peaks.

Figure 2 and 3: The red legend is barely distinguishable from the underground.

Line 147/148: In my opinion, the findings regarding the lattice spacings do not math between TEM and XRD.

Figure 4: In general: XP spectra are plotted from high to low BE on the x-axis. EDS: How is the mentioned atomic ratio visible in the spectrum? Was peak fitting performed?

Figure 5 and following figures: Please assign colors to the samples and graphs instead of naming it a, b etc. That is confusing.

Line 185: “ratios of catalysts”: Is VC really a catalyst? If yes, what is its activity without the addition of MoS₂?

Line 190: “well-defined oxygen reduction peaks” What is meant by that? I do not understand how they refer to the ORR? To me this is a redox transition of the material. Please discuss.

Line 194/195: Generally, the ORR activity should be assessed under rotation (1600 rpm) and not by peak currents from the CV. Next, where is the comparison with literature to justify the improved “activity”?

Figure 7 + discussion on it: A state-of-the-art reference is needed for comparison of the activity.

Figure 9: Please normalize the currents in A und B to the electrode area. Second, why is the annotation of A-D colored in red?

Figure 11 + discussion on it: I highly doubt that the MoS₂ outperforms Pt catalyst in acidic environment. What kind of Pt reference was used? Was the mass activity at 0.9 V_RHE (as commonly reported in literature) determined? Furthermore, state-of-the-art Pt catalyst has a much higher selectivity. Please compare to literature, there a plenty of studies under these conditions.

Figure 12 + discussion on it: First, in Nyquist plot, both axes have to be scaled identically. Second, the semicircle includes high to low frequencies, not only the high frequency range. Third, is it valuable to assume that only charge transfer processes are happening here? At what potentials were the EIS performed and what is happening at that potential?

Figure 14 + discussion on it: Usually for FC application the stability is assessed by a cycling protocol in application relevant potential windows. So, how does the chronoamperometric make sense in this respect? At what potential is it recorded? And why was it only done for the alkaline conditions?

Figure 15 + discussion on it: Why was the potential range of 0.4-1.2 VRHE chosen for this protocol? How does it refer to fuel cell application?

Line 330: How do you know that you have “abundant Mo edges” as active sites? Which method shows that?

 

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

In this work, the authors show flower ball-like MoS2/Vulcan XC-72R (VC) nanocomposites via the hydrothermal method using reductant and morphology control reagents. The designed composite exhibits a nearly 4e- ORR process with 0.78 and 0.92 18 V onset potentials in 0.1 M KOH and HClO4, respectively. Furthermore, the flower ball-like composite shows utmost electrochemical stability judging by 87% and 80% current retention for 20000 s either in alkaline or acid media, long-term durability for continuous 10 000 cycles, and stronger resistance to methanol than the commercial Pt/C catalyst. Abundant Mo edges as catalytic active 22 centers of flower ball-like structure, high electron conductivity, and enhanced mass transport in either alkaline or acidic electrolyte are favorable for catalytic performance. After addressing some of the issues mentioned below, this article can be accepted for publication in this journal.

1. Authors describe the electrochemical properties of  MoS₂  clearly in the introduction section but the introduction of Vulcan XC-72R (VC) and its electrochemical properties are insufficient, hence need one more paragraph about this.
2. In the experimental section authors should add one subheading of materials including all the reagents and chemicals used in this work with their details.
3. In the experimental section 2.1 author should cite the reference they used for the synthesis of MoS₂.
4. In Figure 12, the authors should add one figure of inset on the scale 100/100 (Z'/Z").
5. In Figure 14, the Chronoamperometric curve of Vulcan XC-72R (VC) is not significant because the authors did not include its other electrochemical data such as CV, and polarization curves (If you have, need to add in Figures 6 and 7 for the comparison).
6.  In some of the figures the current (I/mA)  and in some current density (J/ mA.cm^-2) of polarization curve is presented, which is a bit confusing.
7. In comparison to the 0.1 M KOH as an electrolyte 0.1 M HClO₄ shows a very low current from RRDE as shown in Figure 11 (A), why?
8. For a research article 15 figures are too high and the authors do not include any electronic supplementary information (ESI) file. So I would like to suggest transferring some figures to the ESI file. 
9. Furthermore, the individual ORR polarization curves of different modified electrode: (a) MoS2/C-15, (b) MoS2/C-20, (c) 209 MoS2/C-25 and (d) MoS2/C-30 in O2-saturated 0.1 M KOH and 0.1M HClO4 solution at different RPM should be added to ESI.
10. In line 293 "0.1M HClO4" needs correction.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I see the improvements in the reviewed manuscript. However, there are still a few points that the authors should discuss and consider more intensively. Some of the answers were quite short and I missed the support of the statements by literature.

Here are a few points:

• Commercial Pt/C catalysts show activities around 0.2 A/mg_Pt. That is common sense in literature (for example https://doi.org/10.1016/j.jpowsour.2018.04.084). So the authors should reach at least an activity in that range with their reference system. If not, there must be some issues with the experiments.
• The authors should perform an intense literature research on EIS measurements and its interpretation. To me, the approach and results as well as the answers from the review are not consistent. Please compare to literature.
• Stability should be discussed related to fuel cell application. For example, the DoE specifies fuel cell stability protocols that are also commonly accessed for thin-film RDE studies in catalyst development. In there, among others, relevant potential ranges are reported. So the authors should at least compare there stability protocol and comment on main differences.
• The language still has to be improved. Please check carefully.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf