Preparation of High-Efficiency Fe/N-Doped Carbon Catalysts Derived from Graphite Phase Carbon Nitride for Reduction of Oxygen

Round 1

Reviewer 1 Report

Comments to the Authors

The Manuscript entitled “Preparation of high-efficiency Fe/N-doped carbon catalyst derived from graphite phase carbon nitride for reduction of oxygen” by Wang et al. discusses about the development of Fe-NC based catalyst and studied the application of the catalyst for oxygen reduction reaction (ORR). The developed catalyst exhibited significantly high electrocatalytic performance for ORR. Overall the manuscript is well arranged and the following points need to be addressed before this article can be published. The comments to the authors are listed below.

1)      The rationale behind choosing the ex-citu technique compared to the well adopted in-situ technique (10.1002/admi.201900273) for synthesizing Fe-NC should be discussed.

2)      The experimental section is not clear and the authors are requested to provide more details such as annealing atmosphere, ramp of the purging gas etc. Moreover, in experimental part it is claimed that the Fe-NC was synthesized by heating at 800 ⁰C for 4 h, however in the Figure 1 it is different. Please check and rectify the issue.

3)      Figure 4 should be modified since 4a is Raman spectra not XRD. In the discussion of Raman spectra, the authors claimed that the catalyst with higher defects (I_D/I_G) may serve as catalytic active center but the reverse trend was observed for ORR. The authors have to shed light on this issue.

4)      From the XRD pattern the designed Fe-NC catalysts match more with Fe₃C and Fe compared to Fe₃N. However, based only from the HR-TEM (Figure 3d) the authors assigned it to Fe₃N. This needs to be explained. 

5)      Please rectify Figure 6, since the assignment of 6a and 6b is missing. The role of N-doping for ORR is not explained. Please check the following article (10.1021/acsaem.2c02991) and discuss the role of N-doping.

6)      The introduction part is short of information and to enhance the introduction section, consider referencing the following papers: (i) 10.1016/j.nanoen.2022.108020; (ii) 10.1039/D3TA02712K; (iii) 10.1016/j.ensm.2023.102890; (iv) 10.1016/j.cej.2024.149529; (v) 10.1039/D2MH01067D.

Comments for author File: Comments.pdf

Author Response

Comments 1: The rationale behind choosing the ex-citu technique compared to the well adopted in-situ technique (10.1002/admi.201900273) for synthesizing Fe-NC should be discussed.

Response 1:Thanks to the valuable questions raised by the reviewers. The main reason for our use of the ex-situ synthesis technique is the simple and common preparation process.

Comments 2: The experimental section is not clear and the authors are requested to provide more details such as annealing atmosphere, ramp of the purging gas etc. Moreover, in experimental part it is claimed that the Fe-NC was synthesized by heating at 800 ⁰C for 4 h, however in the Figure 1 it is different. Please check and rectify the issue.

Response 2:Thanks to the valuable questions raised by the reviewers. The heating and annealing processes of the experiments were carried out under argon atmosphere, in addition, Fe-NC was synthesised by heating at 800 °C for 2 h. We have modified the experimental parts, and the modifications have been highlighted in red.

Comments 3: Figure 4 should be modified since 4a is Raman spectra not XRD. In the discussion of Raman spectra, the authors claimed that the catalyst with higher defects (I_D/I_G) may serve as catalytic active center but the reverse trend was observed for ORR. The authors have to shed light on this issue.

Response 3:Thanks to the valuable questions raised by the reviewers.A large number of literatures have reported that carbon surface defects are the active sites of ORR. In our study, in addition to surface defects, there are metals and their associated active centers, and the active effect of metals may be stronger than that of defects.

Comments 4: From the XRD pattern the designed Fe-NC catalysts match more with Fe₃C and Fe compared to Fe₃N. However, based only from the HR-TEM (Figure 3d) the authors assigned it to Fe₃N. This needs to be explained.

Response 4:Thanks to the valuable questions raised by the reviewers. We carefully measured the lattice size in HR-TEM and found that the corresponding lattice size should be Fe₃C, not Fe₃N.We have corrected it in the manuscript, and the changes have been highlighted in red.

Comments 5:Please rectify Figure 6, since the assignment of 6a and 6b is missing. The role of N-doping for ORR is not explained. Please check the following article (10.1021/acsaem.2c02991) and discuss the role of N-doping.

Response 5:Thanks to the valuable questions raised by the reviewers. We have modified Figure 6 and after reviewing the literature, the role of N-doping is discussed and highlighted in red in the text.

Comments 6:The introduction part is short of information and to enhance the introduction section, consider referencing the following papers: (i) 10.1016/j.nanoen.2022.108020;(ii) 10.1039/D3TA02712K;(iii) 10.1016/j.ensm.2023.102890;(iv) 10.1016/j.cej.2024.149529; (v) 10.1039/D2MH01067D.

Response 6:Thanks to the valuable questions raised by the reviewers.Based on the reviewers’ comments, we have cited these high-quality literatures.

Reviewer 2 Report

In the paper under review the Fe-NC electrocatalysts for oxygen reduction reaction (ORR) were obtained and characterized. The authors proved the electrocatalysts' ability of O2 reduction, their stability and methanol resistance, revealed the four-electron mechanism of ORR. The advantages of the synthesized catalysts can be explained taking into account such features as rich N-content of the g-C3N4 obtained by melamine pyrolyzing, as well as large amount of defects and hierarchical distribution of micro- and mesopores.

There are several comments on the paper which are recommended to be accounted before the publication of the manuscript.

1. Introduction. The emphasis is on zinc-air batteries, although the ORR process under study is also used in other electrochemical energy conversion devices. It is recommended to make changes to this section, expanding the scope of the problem under consideration.

2. Please show the novelty of the Fe/N-doped carbon synthesys method in more detail, comparing with the known and used ones, and making the emphasis on the advantages of the proposed procedure.

3. Lines 246 "In electrochemical reactions, the greater the active surface area, the greater the reaction rate and current density.". Lines 269-270. "...the relatively high specific surface area also increases the exposure of the active site and thus enhances the catalytic activity." The question is how the every current densities were calculated in the work? The greater the active surface area, the greater the current, but not the current density. Is the observed "catalytic" effect retained after current normalization (i.e. dividing) by the electrochemically active surface area?

4. There are several typos in the manuscript, for instance, Line 44 pt-based instead of Pt-based, Line 150 mesoporous instead of mesopores, Line 209 ". doping" insted of ". Doping", Line 316 Levvich instead of Levich, Line 319-320 diffusion current, kinetic current instead of diffusion current density and kinetic current density. Please check the text.

Author Response

Comments 1:The emphasis is on zinc-air batteries, although the ORR process under study is also used in other electrochemical energy conversion devices. It is recommended to make changes to this section, expanding the scope of the problem under consideration.

Response1:Thanks to the valuable questions raised by the reviewers. Based on the reviewers’ comments, we have extended the introductory section on zinc-air batteries, and the extensions have been highlighted in red.

Comments 2:Please show the novelty of the Fe/N-doped carbon synthesys method in more detail, comparing with the known and used ones, and making the emphasis on the advantages of the proposed procedure.

Response2:Thanks to the valuable questions raised by the reviewers.In fact, we use a relatively simple and common preparation method, mainly using the high nitrogen content of g-C₃N₄. 

 Comments 3:Lines 246 "In electrochemical reactions, the greater the active surface area, the greater the reaction rate and current density.". Lines 269-270. "...the relatively high specific surface area also increases the exposure of the active site and thus enhances the catalytic activity." The question is how the every current densities were calculated in the work? The greater the active surface area, the greater the current, but not the current density. Is the observed "catalytic" effect retained after current normalization (i.e. dividing) by the electrochemically active surface area?

Response3:Thanks to the valuable questions raised by the reviewers.After reviewing the literature, it was found that there is no direct relationship between the active surface area and the current density, and there is no literature support for the statement that the larger the active area, the higher the current density, which is a writing error, and has been corrected in the manuscript and highlighted in red.

Comments 4:There are several typos in the manuscript, for instance, Line 44 pt-based instead of Pt-based, Line 150 mesoporous instead of mesopores, Line 209 ". doping" insted of ". Doping", Line 316 Levvich instead of Levich, Line 319-320 diffusion current, kinetic current instead of diffusion current density and kinetic current density. Please check the text.

Response4:Thanks to the valuable questions raised by the reviewers.We examined the whole text in detail and corrected any typos in the text, with the corrections highlighted in red.

Round 2

Reviewer 2 Report

The authors have taken into account the comments, so the manuscript can be published in the present form.