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Reviewer #1: The authors have synthesized nitrogen-doped reduced graphene oxide@chromium oxide (NRGO-Cr2O3) nanocomposite and used the material as the electrocatalyst in Zn-air batteries. This work lacks innovation and does not analyze the mechanism in sufficient depth. It is not recommended to publish in this journal.
1. What are the catalytic active sites of NRGO-Cr2O3 for ORR?
2. More detailed descriptions need to be provided regarding the relationship between structure of NRGO-Cr2O3 and its ORR performance.
3. The quality of TEM needs to be improved to prove that Cr2O3 nanostructures uniformly distributed on NRGO nanosheets.
4. Why is the LSV curve from the Figure 6A not flat in the low voltage region?
Reviewer #2: Comments to Author.
The study presented an electrocatalyst comprising a nitrogen-doped reduced graphene oxide@chromium oxide (NRGO-Cr2O3) nanocomposite, demonstrating superior oxygen reduction reaction (ORR) activity in alkaline media compared to NRGO and Cr2O3 individually. This research introduces a novel nanocomposite (NRGO-Cr2O3) for application as the air cathode in a zinc-air battery (ZAB). However, several aspects require further detailed investigation.
1. The schematic representation of the synthesis process for the NRGO-Cr2O3 nanocomposite effectively illustrates the synthesis method and experimental procedure. Nonetheless, it contains an error in the chemical formula of chromium nitrate hydrate and an incorrect superscript notation for Celsius.
2. In Figure 2, the magnification levels of the SEM and TEM images have been incorporated, and the labels have been omitted.
3.The typography within the image requires modification.
4.The XRD analysis primarily indicates the presence of Cr2O3 nanoparticles, as well as pure Cr2O3 and NRGO-Cr2O3 composites. Further characterization of the XRD results is necessary to comprehensively analyze the crystal structure and composition, including the identification of chromium oxide phases such as α-Cr2O3, CrO2, and CrO3.
5. How can the short-range ordered structures of NRGO, Cr2O3, and their combined NRGO-Cr2O3 nanocomposites be comprehensively validated?
6.The paper indicates that the grain size enlarges when NRGO is coated with Cr2O3 to form an NRGO-Cr2O3 composite catalyst. This observation prompts the question of how such an increase in grain size can be empirically validated. Furthermore, it is essential to elucidate the mechanisms by which an increased grain size positively correlates with enhanced cell performance.
7.In Figure 6B, the rationale for selecting the potential range of 0.5V to 0.65V warrants clarification. Additionally, an explanation is required for the observed decrease in the number of transferred electrons as the potential increases within this specified range.
8. The stability test for Figure 6B, which includes NRGO, Cr2O3, and NRGO-Cr2O3, is limited to 2000 seconds. This duration may not be sufficient to comprehensively demonstrate long-term stability.
9.Figure 8A, which illustrates the discharge voltage profiles at varying current densities, requires modification.
10. The specific capacity is reported to be 13 mA·h; however, no corroborating evidence or additional information is provided to substantiate this claim.
Reviewer #3:
1.The abstract is informative but could be more concise. Consider rephrasing to eliminate redundancy, especially regarding performance like power density and open-circuit voltage.
2.The long-term cycling stability test is a strong point of the study. However, discussing the degradation mechanisms, if any, observed over 1680 cycles would enhance the understanding of the catalyst's robustness.
3.The EIS data is well-presented, but the discussion could delve deeper into how reduced charge transfer resistance correlates to improved ORR performance.
4.The references are comprehensive, but a few key citations appear outdated. Consider replacing older references with recent high-impact publications in the field of electrocatalysis and zinc-air batteries. These research might be cited as follows: Rare metals, 2023(42), *******; Rare metals,2024 (43), *******; Journal of Energy Storage, *******), 114168; Nanoscale, 2024,16, *******.The results are compared with Pt/C and other catalysts, but the discussion on the performance improvement mechanisms could be expanded. Specifically, highlight how the synergy between NRGO and Cr2O3 contributes to the superior ORR activity.
6.It is necessary to characterize the measurement of sample by TEM for observing the structure of NRGO and Cr2O3, which could clarify the effect between NRGO and Cr2O3.