Importance of Doping Sequence in Multiple Heteroatom-Doped Reduced Graphene Oxide as Efficient Oxygen Reduction Reaction Electrocatalysts

Round 1

Reviewer 1 Report

This manuscript reports on the systemic investigation on the (N,P,B)-doped RGO for the applications of ORR and HER. The content of this work is interesting, but requires revisions before its possible publication.

1. In Fig. 1, it is unclear that the NPBRGO was resulted from single-step or two-step.
2. In Fig. 1(i), the results were inconsistent with the quite low concentration (ca.1 at%) of phosphorous.
3. In XPS results, it is clear that B 1s and P 2s peaks are exactly overlapped between 185 and 195 eV. Therefore, the fitting of the B 1s in In Fig. 2d was incorrect. The influence of P 2s cannot be excluded.
4. In Fig. 2c, the S/N ratio of the single-step sample was quite poor, which led to the incorrect fitting. Please confirmed again.
5. In Fig. 4c-d, the HER test was also performed for as-prepared samples. However, there is the lack of any mentions in the Abstract/Introduction/Conclusions parts.
6. The stability tests of the NPBRGO for ORR and HER should be provided to the readers.
7. Recently, modified/functionalized carbon materials have been good models to enhance oxygen electrocatalysis. Some related references should be read and can be cited in the Introduction section. ( e.g. Nature Reviews Materials 2016, 1, 16064; Journal of Colloid and Interface Science 2021, 600, 865; Applied Catalysis B: Environmental 2021, 285, 119846)

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Reviewer 2 Report

The authors report the effect of doping sequence in ternary heteroatom-doped RGO for use as the ORR catalysts in an alkaline electrolyte. They found that the doping sequence had a significant effect on the bonding configuration and doping content of heteroatoms in ternary-doped RGO. After I have gone through this manuscript with careful evaluation, I think this manuscript cannot be accepted as its present form. Some results are still unclear and need to be clarified. Additional measurements are also required to support their results. The comments and suggestions are listed below.

1. The main point of this work is to prepare the ternary heteroatom-doped RGO with different doping sequences as “ORR electrocatalyst”. However, in the last section of Results and Discussion, the authors reported the HER activity of the electrocatalysts, which had no mention in both abstract and conclusion. I recommend the authors removing the HER activity and only focusing on the ORR activity in this manuscript.
2. The previous reports on the effect of doping sequence on the ORR activity of binary and ternary-heteroatom-doped carbons should be given and cited in the introduction.
3. The first paragraph of the Results and Discussion section contains detailed information on the preparation of ternary-doped RGO, which was already described in section 2.1. I think the authors should remove this paragraph and combine it with section 2.1.
4. The surface area and pore structure of electrocatalysts are factors that play an essential role in influencing the ORR activity of carbon-based electrocatalysts. Therefore, I suggest the authors performing the additional characterization on the surface area and porosity using the N₂ sorption analysis.
5. The catalyst loading of electrocatalysts and commercial Pt/C should be given in section 2.3.
6. From the K-L equation, 0.62 is a constant used to express the rotation speed in rad/s, NOT rpm.
7. The authors described the weight loss of RGO in the second paragraph of page 4. However, there was no TGA result to support this description.
8. From Figure 1, I am curious why the carbon-network layer of the copper grid contained the strong signals of B and N.
9. The XRD measurement should be shown along with the Raman spectra to evaluate the structural properties of the samples.
10. Even though the ID/IG ratio is usually used as an indicator of the degree of disorder of carbon materials, other authors have proposed a better way to measure the degree of disorder in carbon materials from Raman measurements as ID/(ID+IG) and wD vs ID/(ID+IG) (Carbon 32 (1994) 1523; Carbon 98 (2016) 411), which can be more sensitive than the simple ID/IG ratio to classify carbon materials. I suggest the addition of such parameters and compare with the already present in the manuscript.
11. The atomic content of N in Table S1 is wrong. It should be 10.21 atom%.
12. What is the bonding configuration of phosphorus for the single-step samples in Table S1?
13. It seems that one sub-peak at 131 eV was missing from the graph.
14. Please check the peak position of each bonding configuration for both single-step and two-step samples shown in Figures 2b, 2c, and 2d. The same bonding configuration should be located at the nearly same position for single-step and two-step samples. For example, the oxidized-N peak of the single-step sample should not be located at the same binding energy of quarternary-N of the two-step sample. The peak position of all bonding configurations with References should be added in the XPS results.
15. The LSV and electron transfer number of NPBRGO (single-step), NPBRGO (two-step), NPRGO, BRGO, and RGO should be plotted in the same figure for better comparison.
16. The authors should compare the ORR activity (current density and onset potential) of ternary-doped RGO obtained in this study with those reported in the literature.
17. The CV curves of NPBRGO catalysts prepared with single-step and two-step doping should be shown together for comparison.
18. The authors stated that “NPRGO and NPBRGO show more efficient one-step four-electron pathway to directly reduce O2 to OH-“. I do not agree with this conclusion. As can be seen, the LSV curves of both NPRGO and NPBRGO did not show the plateau behavior in the diffusion-controlled region. Moreover, the continuous increase in current density was observed at a lower potential. These two behaviors may indicate a combination of two- and four-electron pathways involving in the ORR process.
19. The flow of explanation of electrochemical results should be rearranged. It should be arranged in the following order: CV > LSV > RDE > K-L > electron transfer.
20. There was no stability test of NPBRGO in the manuscript. To confirm the ORR stability under long-term operation, the authors should measure the current-time response or the LSV before and after the accelerated cycling test of the NPBRGO and compare it with the Pt/C.
21. Please correct the subscript errors; for example, O2, B2O3, etc.

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Reviewer 3 Report

The paper reports a systematic study of the multiple heteroatom-doped (N, P, and B) graphene oxide as efficient oxygen reduction reaction (ORR) electrocatalysts. The most important conclusion is that they believe the doping sequence (one-step or two-step process as wrote) determine the different efficiency towards ORR, because different amounts of dopant will be added in two processes.   Although many persuasive experiments have been carried out to support the authors’ conclusions. However, as a reviewer, I did not exactly know the most promising amount ratios of P, N, and B atoms (to get higher conversion efficiency for ORR) from this paper. In addition, what is the most promising doping position for these dopants? (Are they in graphitic, pyridinic, or pyrrolic, etc)?  

 

There, some minor revisions are required to add to the value of the manuscript for definitely supporting its acceptation. Overall, I rate this manuscript as scientifically interesting for the audience of applied nano

 (1) The main question has been provided above. Please tell the audience more clearly of your structural data. What is the most promising amount ratios of P, N, and B atoms for ORR?  what is the most promising doping position for these dopants? (Are they in graphitic, pyridinic, or pyrrolic, etc)?  

 

(2) Why do you prefer to use DNA as the source for N and P dopant? Using DNA to produce graphene-dopant, interesting but a little bit wired. Is there any other similar method used in other literatures to support your paper?

 

(3) The difference of Figure 3 (b) and (c) is not obvious for audience. I can only know the difference from texts “ We observed a higher onset voltage for the two-231 step synthesized RGO (ca. 0.89 V) compared to single-step synthesized RGO (ca. 0.8 V), 232 indicating the substantial increase in the availability of ORR active sites due to the pres-233 ence of the high proportion of P atoms with the help of B atoms. (page 6)”

       Actually, I do not think there is significant difference between 0.8 V and 0.89 V towards the ideal equilibrium potential 1.23 V. But maybe it is OK to publish.

       Second, the color of 2500 rpm in (b) and (c) are different. What does that mean?

 

(4) “while NPRGO and NPBRGO show more efficient one-step four-electron pathways to directly reduce O2 into OH− [35]. (page 8)”

        

       This sentence is controversial. In the mechanistic studies, it usually divided into 4 steps for four electron transfer steps (J. Phys. Chem. B 2004, 108(46),17886-17892.; Nat. Catal. 2018, 1(5), 339; J. Phys. Chem. C 2019, 123, 50, 30335–30340). The above three refs are suggested to be cited for audience to know the mechanisms of four-electron pathways towards ORR/OER on the graphene with dopant.

 

(5), Please revise your conclusion (together with the comment #1). In the current version, the conclusion is not clear, and seems there are many uncertain discussion in it (such as There are four possible means to explain high ORR electrocatalyst ternary RGO using two step doping: (a) …,(b)…, (c)…, and (d)).  

       Please tell the audience what the most important finding is in your paper. Discussions are not required in the final conclusion.

Author Response

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Round 2

Reviewer 1 Report

Relative to authors’ previous work (published in New. J. Chem. 2016, 40, 6022–6029.), this work lacks novelty. In particular, this work showed the poor performance of ORR than previous work. Most important, many of the reviewers’ comments were not answered in a detail. For examples: The doping position for P, N, and B atoms? The effect for P 2s in Fig 2b? The stability tests for ORR? Inconsistency of elemental mapping with Table S1. Therefore, I would not recommend the publication of this paper in Applied Nano.

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Reviewer 2 Report

The authors have made the appropriate revision and response according to my comments. This manuscript can be accepted after minor revision.

1. There are still some typos and subscript errors, such as "Tw step", B2O3, C mol-1, etc. Please carefully check both the manuscript and supporting information.
2. The unit of bulk concentration of O2 is wrong. It should be mol/cm³.
3. Please also check the unit of kinetic viscosity.
4. It seems to have one more peak at higher binding energy for the N 1s spectrum of the two-step sample. Should the pyridinic-N oxide species be taken into the account for consideration?

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Reviewer 3 Report

Publish as it is.

Author Response

We appreciate the reviwer's positive comment. 

Round 3

Reviewer 1 Report

The authors claimed that the purpose of this work is to understand how the doping sequence affect the electrocatalytic performance. In particular, the doping sequence should be optimized to maximize the electrocatalytic performance of ternary doped graphenes. However, the best performance of ternary doped graphenes cannot be confirmed roughly by using either single-step or two-step processes. Therefore, the effects of other doping sequence must be compared in detail and provided to the readers. If so, the impact of this manuscript would be improved greatly.

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Reviewer 2 Report

This manuscript can be accepted for publication in Applied Nano.

Author Response

We appreciate the reviewer's positive comment.