[NiFe]-(Oxy)Sulfides Derived from NiFe₂O₄ for the Alkaline Hydrogen Evolution Reaction

Round 1

Reviewer 1 Report

The present paper deals with the novel synthesis approach and characterizations of bimetallic Fe/Ni (oxy)sulfide (NFS) materials prepared by the controlled sulfidation of NiFe₂O4 (NFO) and further investigating their electrochemical HER performances. The influence of the H₂S gas composition and the synthesis temperature on the sulphur incorporation to NFS materials and consequently on their HER performance have been explored. The material and reaction described in this study will be of interest to the community of researchers working on the metal sulfide catalysts and HER reaction. Though the paper has quality that I would expect from a paper in “Energies”, few modifications/revisions should be made before being accepted for publication, considering the following comments:

• In section 2.1, it is important to precise all the vendors and products purity
• Before using abbreviations, it is necessary to provide their full forms during its first occurrence. For example, the expanded forms of RHE and LSV have not indicated even once. Check all such abbreviations in the paper.
• XRD part in Section 3.1: The JCPDS and/or other suitable references should be provided for the NFO and two NFS phases as indicated in the figure 1. Also, the peak indices in the XRD are not legible in figure 1, hence this figure should be replaced with a high quality one. Finally, it is evident from the diffractograms that the temperature has an influence on the crystallinity, it would be good to comment on this point.
• The current results of XPS are very brief so authors should extract more details from it. For example, Fe peak is observed not only at 710 eV but also at around 723 eV. What is the reason for exhibiting these two peaks? Moreover, peaks are broad which needed a deconvolution to extract more details. For example, Fe peak at 710 eV is too broad (between 717 - 707 eV) and appear to be resulted from two peaks, elaborate more on this? Similarly, XPS of all element peaks need to be verified and elaborated with reason.
• SEM results: From the current SEM images with a micrometric scale, it is difficult to make conclusions if the particles are spherical shape and nanometer sized. It would be better if authors can show zoomed high-quality image of at least one sample, one each from NFS-H₂ and NFS-N₂ series to confirm this. Moreover, a zoomed SEM image can help to fully confirm the agglomeration phenomena.
• HER performance: It seems that the HER activity results can be related to the XPS surface S or other elements? Because it appears from the S 2p XPS peak of NFS-N₂ that surface S species are present more at low temperatures (150 & 200°C) compared to higher temperatures. In contrast, surface S species are hardly observed at lower temperatures (100 to 200°C) in NFS-H₂ but present in comparatively large amounts at higher temperatures (250 & 300°C). That is why it is important to deconvolute the XPS peaks as mentioned before. Perhaps, authors can try to correlate peaks intensities (or ratios) of other elements to the HER activity.

         Moreover, authors mentioned that S:M ratio is around 1:1 for all NFS-H₂ (see lines 195-200) and presence of the pyrrhotite phase seems to play a major role for an increased HER activity. However, from XPS it seems that the surface S:M ratios of the NFS-H₂ materials are different which may also influence the performance in addition to formation of pyrrhotite phase. This can be confirmed only by deconvoluting and extracting more details from XPS. Hence, authors should explain these results carefully.

• The legends in the figure 2 are not legible. Also, the decimal points in the x-axis values in the figures are separated by commas which should be changed to points. Perhaps, it would be better to separately plot chronopotentiometry data (Figures 2g – 2i) from electrochemical performance data figures.
• Cyclic voltammograms used to calculate the double layer capacitance should be included in the supplementary part.
• English revisions required: Authors should revise the sentence structuring and grammar thoroughly before re-submission. For example, lines 17-18 & 180-183 are not well structured. Also, authors have used mixed tenses (present & past) in many texts, clearly evident in the conclusion part.

Author Response

The authors thank the referee for taking the time to evaluate our manuscript and for their valuable suggestions for improvements. With this submission we are convinced to provide an overall improved version of our work which suits the requirements for Energies. In detail, we addressed the individual comments as follows:

 

1. In section 2.1, it is important to precise all the vendors and products purity.

 

We have included the vendors and product purity in section 2.1.

 

2. Before using abbreviations, it is necessary to provide their full forms during its first occurrence. For example, the expanded forms of RHE and LSV have not indicated even once. Check all such abbreviations in the paper.

 

We have carefully checked the manuscript and provided the full forms before using the respective abbreviations.

 

3. XRD part in Section 3.1: The JCPDS and/or other suitable references should be provided for the NFO and two NFS phases as indicated in the figure 1. Also, the peak indices in the XRD are not legible in figure 1, hence this figure should be replaced with a high quality one. Finally, it is evident from the diffractograms that the temperature has an influence on the crystallinity, it would be good to comment on this point

 

We now have cited references for the NFO and NFS reference patterns. Furthermore, we have adjusted the indices and added a comment regarding the crystallinity in the manuscript.

 

4. The current results of XPS are very brief so authors should extract more details from it. For example, Fe peak is observed not only at 710 eV but also at around 723 eV. What is the reason for exhibiting these two peaks? Moreover, peaks are broad which needed a deconvolution to extract more details. For example, Fe peak at 710 eV is too broad (between 717 - 707 eV) and appear to be resulted from two peaks, elaborate more on this? Similarly, XPS of all element peaks need to be verified and elaborated with reason

 

The region at around 723 for the iron originate from the 2p_1/2 peak. To make the graph more comprehensible we have adjusted the scale, so that only the 2p_3/2 peak is visible, similar to the Nickel measurements. Furthermore, we noticed, that a normalization of the XPS data was not performed, therefore, we provide normalized XPS data as well, which enables an improved comparison of the data. Additionally, we have performed a deconvolution of 2 representative samples, which is included in the SI.

 

5. SEM results: From the current SEM images with a micrometric scale, it is difficult to make conclusions if the particles are spherical shape and nanometer sized. It would be better if authors can show zoomed high-quality image of at least one sample, one each from NFS-H₂and NFS-N₂series to confirm this. Moreover, a zoomed SEM image can help to fully confirm the agglomeration phenomena

 

We have included images with a lower and higher magnification in comparison to the already provided images. We hope that these help to confirm the agglomeration of the particles.

 

6. HER performance: It seems that the HER activity results can be related to the XPS surface S or other elements? Because it appears from the S 2p XPS peak of NFS-N₂that surface S species are present more at low temperatures (150 & 200°C) compared to higher temperatures. In contrast, surface S species are hardly observed at lower temperatures (100 to 200°C) in NFS-H₂but present in comparatively large amounts at higher temperatures (250 & 300°C). That is why it is important to deconvolute the XPS peaks as mentioned before. Perhaps, authors can try to correlate peaks intensities (or ratios) of other elements to the HER activity.

 

7. Moreover, authors mentioned that S:M ratio is around 1:1 for all NFS-H₂(see lines 195-200) and presence of the pyrrhotite phase seems to play a major role for an increased HER activity. However, from XPS it seems that the surface S:M ratios of the NFS-H₂materials are different which may also influence the performance in addition to formation of pyrrhotite phase. This can be confirmed only by deconvoluting and extracting more details from XPS. Hence, authors should explain these results carefully.

 

6. and 7.: Although we have made a deconvolution of the XPS-data, we see pronounced S peaks for NFS₃₀₀-H₂ and NFS₁₀₀-N₂, which show the best performance. However, a pronounced S peak is also seen for the NFS₂₀₀-N₂material, which is one of the worst performing catalysts in this series. We believe that correlating the activity solely with the S surface species would be too simplistic. For this, more complex and theoretical investigations are necessary, which are, however, beyond the scope of this manuscript but will be now elaborated further.

 

8. The legends in the figure 2 are not legible. Also, the decimal points in the x-axis values in the figures are separated by commas which should be changed to points. Perhaps, it would be better to separately plot chronopotentiometry data (Figures 2g – 2i) from electrochemical performance data figures.

 

We have adjusted the decimal points and have plotted the chronopotentiometry data separately.

 

9. Cyclic voltammograms used to calculate the double layer capacitance should be included in the supplementary part.

 

We have included 2 representative CV measurements, one for NFS-N₂ and one for NFS-H₂, in the supplementary part.

 

10. English revisions required: Authors should revise the sentence structuring and grammar thoroughly before re-submission. For example, lines 17-18 & 180-183 are not well structured. Also, authors have used mixed tenses (present & past) in many texts, clearly evident in the conclusion part.

 

We have carefully checked the manuscript and have restructured sentences accordingly.

Reviewer 2 Report

The manuscript presents a novel synthetic procedure of Fe/Ni (oxy)sulfides i.e. by sulfidation of NiFe2O for hydrogen evolution reaction. The work is correct, but the originality level is low, and obtained HER results are not outstanding compared to the other literature reports. Even though, the work interestingly presents the effect of the sulfidation process on the HER performance of the catalyst. There are several issues that must be improved in the manuscript: 

• the abstract should contain more details related to the obtained research results; how the sulfidation influenced properties of the catalysts?; which catalysts showed the most efficient electrocatalytic performance? why?; obtained HER overpotential should be presented for 10 mA/cm2 (not only fo 100 mA/cm2),
• the introduction is quite poor in the case of presenting HER performance of other Ni-Fe or other sulfided catalysts,
• Results ad discussion: the quality of Figure 1 is very low; the font in each Figure should be clear and of a size similar to the whole text (the same Figure S1),
• please explain, why in the case of NFS-H2, the Fe:Ni and S:M ratio have become a plateau? why the authors indicated 1.09 as plateau? what means plateau in this situation ?
• Electrochemical study: Figure 2: each graph should be differentiated not only by the colour but also by the pattern (for black and white reading); moreover, the number presented in the x and y axes should be with a point (not a comma); the quality of this figure is also low; 
• LSV, Cdl, and stability test should also present data for the state-of-the-art e.g. Pt/C (the most efficient for HER) for comparison
• the authors presented in the manuscript only the final values for Cdl and ECSA; some examples of cyclic voltammograms (CV) including all scan rates should be presented in the Supplementary Materials e.g. for example for the 2 the most efficient samples i.e. NFS150-N2, NFS300-H2; it is important that the reader can see if the CV for Cdl determination reveals pure capacitive behavior (only then are these calculations correct),
• the chronoamperometry measurements performed for 1 h and 10 h cannot be presented as long-term stability studies; I would say, that these are preliminary stability studies, not long-term ones.
• Discussion: no discussion of the results was presented in the manuscript; the authors should explain the catalytic performance revealed by the catalyst; why NFS150-N2, NFS300-H2 exhibited the higher HER overpotential? what is the role of NiFe and sulfides in the hydrogen evolution reaction? the authors should also compare their final results with the available literature reports; moreover, the table with the similar catalysts and corresponding HER overpotential should be presented in the supplementary or in the manuscript; there are several works on different materials for HER with much better HER overpotential: e.g. NiFeS/Ni foam with 222 mV (X. Shang et al., Electrochim. Acta 256 (2017) 241) or NiFe with 142 mV (Z.Zhang et al., Int. J. Hydrog. En. 2021: 10.1016/j.ijhydene.2021.10.150).  

Author Response

We are more than puzzled about the late incoming review and this refereeing report just showing up just a couple of hours before the deadline for resubmission. Likewise, we are very much surprised about the general comments of the referee: "The work is correct, but the originality level is low, and obtained HER results are not outstanding compared to the other literature reports. Even though, the work interestingly presents the effect of the sulfidation process on the HER performance of the catalyst. "

We thank the referee to state that the work is correct suggesting that there is no scientific error present in this manuscript.

In addition, yet we are very much puzzled about this statement - the aim was not to show a next level high performance catalyst but instead reasonably show the influence of the sulfidation process. As such, with the comment of the referee we seem our aim fulfilled.

Due to the short time, we are however, given to reasonably answer this referee, we cannot give more detailed answers and refer to the answers to the two other referees. 

 

Reviewer 3 Report

Comments to the Author
The authors have reported electrocatalytic performance toward H2 production of NiFe-(oxy) sulfides electrodes prepared by a controlled (partial) sulfidation of NiFe2O4 (NFO) nanoparticles in H2S containing atmospheres. The paper is well written but there are some major flaws in the manuscript, which needs to be addressed, pointed below. 

1. Actually your introduction was well constructed. But, it would be great if you could compare present results with the literature in more detail. I would appreciate, if you could define clearly the superiority, novelty and the critical improvements in your paper (in abstract as well as in the introduction) compared to other similar works. Additional related papers could be cited, (1016/j.jechem.2019.01.017.,10.1016/j.ijhydene.2019.07.034.,10.1002/smll.202105544., and 10.1016/j.arabjc.2018.04.010)
2. How did authors validate the formation of nickelian pyrite (Ni0.35Fe0.65S2) and nickelian pyrrhotite (Ni0.35Fe0.65S) catalysts? Authors should add the XRD card number in Figure 1. Figure 1 need to be reconstructed and captions must be revised accordingly in the manuscript.
3. Figures S2, and S3 More clear discussion on SEM must be provided. Scale bars not visible to the readers and high resolution images must be provided.
4. Has the author removed the dissolved oxygen of the electrolyte by purgin nitrogen/Argon? This is a very common and highly recommended exercise in electrocatalysis reaction related to hydrogen evolution reduction.
5. Did you observe any contamination coming from the Pt counter electrode . Many studies found that " both electrochemical and chemical dissolution of Pt in acidic or alkaline electrolyte cause redeposition of Pt on the working electrode of interest, which significantly improves the HER activity.(one could employ here a graphite electrode instead)?
6. Please include a Tafel plot for NFS_N2, and NFS_H2 electrocatalysts (at least in supporting info) to make a better evaluation of the results. in figure2a, and d, the LSV results for Pt/C catalysts should be provide for a proper evaluation of the HER results.
7. The Nyquist diagrams given in Figure S5 for both NFS_N2, and NFS_H2 electrocatalysts are not adequately constructed because of which they seem elongated. To present Nyquist plots properly, it is necessary that the length of the measure units of the x- and y-axis be the same, i.e. the length of the ohm unit in the x-axis should be equal to the length of the ohm unit in the y-axis. Equivalenet circuit and parameters need to be presented.
8. The reason for the good stability of NFS300-H2 catalysts could be elaborated on page 7.
9. The authors commented on the good stability for NFS300-H2 catalyst. However, the structural stability or the surface structures of these electrodes during or after catalysis is not clear. To confirm the stability, NFS300-H2 electrode should be more clearly characterized after the test , such as TEM, XPS and XRD, and compared with fresh materials.
10. The performance of the NFS_N2, and NFS_H2 electrocatalysts for the production of H2 should be compared with the reported values of the catalysts in the literatures. The performance should be listed in a table, including overpotential, electrolyte, and Tafel values.
11. All the figures need to be reconstructed. Hard to read captions of the figures. Tables format must match with journal style. Author must pay attention to journal templates.

 

Author Response

The authors thank the referee for taking the time to evaluate our manuscript and for their valuable suggestions for improvements. With this submission we are convinced to provide an overall improved version of our work which suits the requirements for Energies. In detail, we addressed the individual comments as follows:

1. Actually your introduction was well constructed. But, it would be great if you could compare present results with the literature in more detail. I would appreciate, if you could define clearly the superiority, novelty and the critical improvements in your paper (in abstract as well as in the introduction) compared to other similar works. Additional related papers could be cited, (1016/j.jechem.2019.01.017.,10.1016/j.ijhydene.2019.07.034.,10.1002/smll.202105544., and 10.1016/j.arabjc.2018.04.010)

 

We have incorporated the proposed papers into the manuscript.

 

2. How did authors validate the formation of nickelian pyrite (Ni0.35Fe0.65S2) and nickelian pyrrhotite (Ni0.35Fe0.65S) catalysts? Authors should add the XRD card number in Figure 1. Figure 1 need to be reconstructed and captions must be revised accordingly in the manuscript.

 

We have cited references for the NFO and NFS reference patterns and have reconstructed Figure 1.

 

3. Figures S2, and S3 More clear discussion on SEM must be provided. Scale bars not visible to the readers and high resolution images must be provided.

 

We have included images with a lower and higher magnification in comparison to the already provided images to provide more information regarding particle agglomeration. Furthermore, we have adjusted the scale bars.

 

4. Has the author removed the dissolved oxygen of the electrolyte by purgin nitrogen/Argon? This is a very common and highly recommended exercise in electrocatalysis reaction related to hydrogen evolution reduction.

 

We are puzzled about this statement. While we agree that nitrogen and argon purging can have an effect on the HER performance, such manipulations are of no technical importance as the catalyst/system has to survive also these conditions and remaining O2 will readily be removed via the oxygen reduction reaction at the cathode. Usually and visible for Pt, there is no significant influence on the HER performance after thorough electrochemical polishing of the electrode. In addition, even when O2 is thoroughly removed from the cathode area, gaseous O2 can still easily diffuse through the membrane. However, to take the referees comment seriously, we checked the performance also upon purging. Notably, no changes are observed under the herein used conditions.

 

5. Did you observe any contamination coming from the Pt counter electrode . Many studies found that " both electrochemical and chemical dissolution of Pt in acidic or alkaline electrolyte cause redeposition of Pt on the working electrode of interest, which significantly improves the HER activity.(one could employ here a graphite electrode instead)?

 

We appreciate and understand the concern of the referee. However, we did not observe any Pt deposition on our WE by XPS and this deposition is unlikely herein due to the separation of the two compartments by using a two-compartment cell including a separator as was already shown in previous work.

 

6. Please include a Tafel plot for NFS_N2, and NFS_H2electrocatalysts (at least in supporting info) to make a better evaluation of the results. in figure2a, and d, the LSV results for Pt/C catalysts should be provide for a proper evaluation of the HER results.

 

We have performed Tafel analysis which can be now found in the SI.

 

7. The Nyquist diagrams given in Figure S5 for both NFS_N2, and NFS_H2electrocatalysts are not adequately constructed because of which they seem elongated. To present Nyquist plots properly, it is necessary that the length of the measure units of the x- and y-axis be the same, i.e. the length of the ohm unit in the x-axis should be equal to the length of the ohm unit in the y-axis. Equivalenet circuit and parameters need to be presented.

 

We have adjusted the x- and y- axis accordingly. Since fitting of EIS data is anything but trivial and there is often over-interpretation of the data, we have decided to show only the raw data.

 

8. The reason for the good stability of NFS300-H2 catalysts could be elaborated on page 7.
9. The authors commented on the good stability for NFS300-H2 catalyst. However, the structural stability or the surface structures of these electrodes during or after catalysis is not clear. To confirm the stability, NFS300-H2 electrode should be more clearly characterized after the test , such as TEM, XPS and XRD, and compared with fresh materials.

 

9. and 10.: We did a purely phenomenological study here to see how sulfidation of NFO affects the HER performance. Since the catalysts here perform only mediocre, we refrain from spending further resources on this point at this point.

 

10. The performance of the NFS_N2, and NFS_H2electrocatalysts for the production of H2 should be compared with the reported values of the catalysts in the literatures. The performance should be listed in a table, including overpotential, electrolyte, and Tafel values.

 

We are aware that such a comparison is often given in literature. Yet and in general, the activity of catalysts depends on many factors such as the preparation method, support, electrolyte, loading and the material making a direct comparison quite tricky and becomes in the worst-case misleading.

Additionally, since there are not many FeNi oxysulfides reported to compare with anyway, we would refer here to the literature we have already cited in the manuscript.

 

11. All the figures need to be reconstructed. Hard to read captions of the figures. Tables format must match with journal style. Author must pay attention to journal templates.

 

We concentrated our efforts to further improve our figures and hope that it meets the expectations of the referee.

 

Round 2

Reviewer 2 Report

The manuscript was not corrected by the Authors as suggested by the Reviewer. 

Author Response

Thank you very much for your comments. We have made desired changes. Please see the attached file.

Author Response File: Author Response.pdf

Reviewer 3 Report

The author has successfully addressed all the comments. I feel the revised manuscript is now well readable and eligible to be published in this journal.

Author Response

Thank you very much.

Round 3

Reviewer 2 Report

The revision has been made.