Fabrication of Conjugated Porous Polymer Catalysts for Oxygen Reduction Reactions: A Bottom-Up Approach

Round 1

Reviewer 1 Report

The present paper entitled “Fabrication of Conjugated Porous Polymer Catalysts for Oxygen Reduction Reactions: A Bottom-up Approach” by the authors Sujoy Bandyopadhyay, Su Ryong Ha, M. Alam Khan, Cheongbeom Lee, Hong In Jeong, Snehal Lokhandwala, Mohaseen S. Tamboli, Bo Ram Lee, Danil W. Boukhvalov and Hyosung Choi describes the synthesis of a new conjugated porous polymer and their use as heterogenous catalyst for the oxygen reduction reaction in alkaline media. A four-electron mechanism is proposed on the basis of DFT calculations.

After careful evaluation of the manuscript, I consider that it would typically appeal to the readers of Catalysts, and therefore I would recommend this paper for publication if authors could address the following comments:

1. At line 77 authors argued that the slightly higher stretching frequency for aromatic C-H bonds with respect the standard presented is enough for justifying the presence of Van der Waals bonds between aromatic rings. Authors do not provide a reference for that standard value, but in any case, the difference is too subtle to reach that conclusion. As Coates described in Interpretation of Infrared Spectra, A Practical Approach. In Encyclopedia of Analytical Chemistry (eds R.A. Meyers and M.L. McKelvy), C-H stretching occurs above 3000 cm^-1 and the position of the band is clearly determined by the type of ring involved, being even more complex when multiple or fused rings are present. As a reference point, authors can check the work of Trivedi et. al in American Journal of Chemical Engineering. Vol. 3, No. 5, 2015, pp. 58-65 where IR C-H stretching values for the byphenyl are located in the same zone as that observed by authors. The evidence of VdW interactions could be obtained from X-ray crystal structure determination. If adequate crystal cannot be obtained, I suggest authors to rewrite the sentence at line 77 “The measured frequency was slightly higher than standard (3030 cm-1) corresponding to the formation of van der Waals bonds aromatic rings in the polymeric material” in a more precise way and also I would suggest to provide an adequate reference for the C-H stretching value indicated
2. Weight loss detected in TGA analysis should be discussed. Although it is not significative it could result confusing considering that the technique has been employed for evaluate the stability of the polymer.
3. Authors have determined a surface area for the polymer of 65 m²/g but this result by itself provide incomplete information for evaluating the catalyst performance. A detailed study of the pore size distribution in the catalyst would be recommendable for completing the textural properties of the material and know precisely their influence in the catalyst activity.
4. Electrocatalytic activity should be discussed by comparing the onset potential with Pt/C benchmark material and also with previous porous polymer obtained by authors (Dyes and Pigments 170 (2019) 107557). How the new structure has influenced to the catalytic activity of the system? Which conclusions have the authors drawn according the change in the structure?
5. I did not find in the article the half-wave potential which is a relevant value for evaluating the catalyst activity.
6. Stability of the catalyst is an important aspect to be considered. In this sense, authors should evaluate its chronoamperometric response by injecting pure methanol into the oxygen-saturated electrolyte.
7. Koutecky–Levich analyses should be included in the supporting information of the article.
8. Authors should update bibliography, there is only two articles from 2019 (one from the own group) and none from 2020.

Author Response

Dear Sir,

 

Please find herewith attached responce to reviewers comments. 

Author Response File: Author Response.pdf

Reviewer 2 Report

In this paper Bandyopadhyay et al. present a combined experimental and computational investigation on conjugated porous polymers for the ORR.

The authors use a variety of experimental techniques, ranging from electrochemical measurements to TGA to assess the electrocatalytic activity and stability of the synthesized polymers. Furthermore, they use a DFT approach to elucidate the reaction mechanism and the energetics of ORR on the polymer. 

Overall, I think that this is a good quality manuscript and potentially interesting for the catalysis community. Electrochemistry in alkaline environment is becoming increasingly popular and works in these area should be emphasized. 

At the same time, I have individuated a couple of details that could elevate even further the quality of this work. For this reason, I think that the manuscript could be published provided the following major revisions, regarding the computational part, are addressed.

1) I suggest to present the computational results in a more systematic form. The energetic of each step is better visualized with free energy diagrams, starting from O2 adsorption and further reduction.

2) The authors hypothesize that the first step of the ORR is the coordination of O2 with two N atoms from the polymer. These two N atoms appear quite distant from each other. I am wondering how this dissociation can take place. Furthermore, the activity of metal-free catalysts towards bond dissociation steps is limited with respect to metallic catalysts. Could the authors comment on this issue, supporting their statement with activation energy barrier calculations? It is probable that the 4e- pathway may take place in an associative way, mediated by OOH.

3) I would suggest the use of the Computational Hydrogen Electrode (CHE) formalism, introduced by Norskov and co-workers to calculate to present free energies at a given potential and calculate the overpotential for the ORR.

4) A clear comparison between the performances of these polymers and state-of-the-art platinum catalysts should be included.

5) Do the authors exclude any possible source of metal contamination deriving from the synthesis of these polymers? Small quantities of Pt/Pd may catalyze the ORR as well.
 

Author Response

Dear Sir,

 

Please see the attachement. 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

It seems that most of the suggestions and comments for the original manuscript have been adequately addressed. In my opinion current version of the manuscript seems to be much clearer for readers, and in my opinion, it can be considered for publication as it is.

Reviewer 2 Report

The authors have addressed my comments.