Review Reports

Round 1

Reviewer 1 Report

In this work, Inês S. Marques et al. constructed mixed metal polyoxometalates@carbonbased composites exhibiting good ORR and OER performances. The manuscript has been well organized and advances the field of electrocatalysts for ORR and OER application. I recommend the publication of the present manuscript after addressing the following minor revisions:

1)  The authors affirm that a ‘possible synergetic effect occurs when the POM presents both metals (Fe and Ni).’ Could they give a more detailed explanation about this effect? This synergistic effect is well known but often not well explained. It could be deepened by the authors.

2) The ƞ10 of 0.36V, reported for the most performant Fe2Ni2@MWCNT_N6 catalyst, is good but, to my knowledge, still higher compared to many other reported electrocatalysts in literature. Do the authors have an idea of how it could be improved? 

3) Have the authors calculated the TOF values? This parameter is important since it better reflects the intrinsic catalytic activity.

4)  In Table S1 reporting the comparison of the OER electrochemical performance authors could add more literature works to compare with.

5) The authors are advised to cite recent published relevant articles focused on OER catalysts, such as Nanomaterials 2021, 11(11), 3010 and ACS Appl. Energy Mater. 2021, 4, 11, 12671–12676.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

In this paper, the authors reported new composite materials based on multi-walled carbon nanotubes doped with nitrogen and Wells-Dawson sandwich polyoxometalates with both metals (Fe₂Ni₂) in order to understand the influence of the metal on the electrochemical performances towards both the ORR and the OER processes. The electrocatalyst outperformed most POM@carbon reported and represents a step forward towards bifunctional electrocatalysts using less expensive materials. However, some major problems should be solved in this paper before it was published in Catalysts. Below are my comments:

1. The author emphasizes that synergetic effects of mixed metal in the title, so how do Ni and Fe interact with each other to improve OER/ORR performance? Is it the change of the electronic states of the metal or the coordination environment of the metal active center?

2. The authors should further in-depth analyze the XPS data, especially the high-resolution spectra of Ni and Fe in the three composites.

3. Please check the data carefully to make sure that the data are reasonable. For example, the OER performance of commercial RuO₂ should be much better than the data provided in figure5a.

4. The paper claims that the electrocatalyst outperformed not only most POM@carbon reported so far as the state-of-the-art RuO₂ electrocatalyst, so, it is recommended that the authors list the reported OER/ORR data of the same type of catalysts and compare with this work.

5. It is necessary to cite new and high-quality literature to enhance the persuasiveness of the paper.

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The paper has been revised according to reviewers' comments. It can be accepted now.