Construction of Flower-like FeCo₂O₄ Nanosheets on Ni Foam as Efficient Electrocatalyst for Oxygen Evolution Reaction

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article " Construction of flower-like FeCo2O4 nanosheets on Ni foam as efficient electrocatalyst for oxygen evolution reaction" is devoted to studying flower-like FeCo2O4 nanosheets supported on Ni foam prepared by hydrothermal+calcination treatments. The following points should be revised:

1.     In the manuscript, the authors wrote: “However, the noble metals because of high price, scarcity and inferior durability have limited their further wide practical applications.”. Then, they propose the use of transition metal oxides as an alternative. However, there are several environmental issues associated with Co.

2.     The purpose of the article is not clearly addressed in the manuscript. The materials are not new, nor is the synthesis method. The authors should better explain the novelty of the work in the introduction.

3.     In the introduction, the authors wrote: “Zhao reported FeCo2O4@FeCo2S4@PPy catalysts through a facile preparation, which exhibited an excellent electrocatalytic performance with overpotential of 98.2 mV for HER at 10 mA 50 cm-2 and 280 mV for OER [11].”. However, “facile preparation” is abstract. Please clarify what the synthesis method was.

4.     Please indicate the signal amplitude of EIS measurements in the experimental.

5.     Figure 1. Data at 160 °C. There is an unrecognizable character in the middle of the pattern. Also, there is an unidentified peal on the left side of the (440) one. Please also indicate the PDF file for the Fe2O3 phase.

6.     In the results, the authors wrote: “Such interconnection of the nanosheets construct flower-like microspheres with a diameter 2 um”. From the SEM images, the diameter is much higher than this value. Can the authors clearly indicate in the micrographs this value?

7.     To support the discussion of the morphology differences with temperature (with a subsequent impact on the electrical properties), surface area measurements are needed (e.g., BET analysis).

8.     The XPS analysis should be revised. The background doesn’t seem correct, as it is cutting the data. Please choose carefully the background function as well as its parameters. Also, please provide the envelope curve fitting the experimental data. In Figure 4c) the envelope is missing, neither all the experimental points are fitted.

9.  The discussion of Figures 5 and 6 is insufficient. The authors should give a proper scientific explanation for the tendency of the results with increasing temperature. Currently, the authors are just describing the results, which is insufficient for a scientific article. Here, a correlation between microstructure and surface area measurements is needed.

10.  The EIS data must be orthonormal, and the frequency (log10) should be added to the points. Please describe the equivalent circuit used and provide fitting parameters with errors.

11.  Table 1 should also compare with other FeCo2O4 samples from the literature.

Comments on the Quality of English Language

There is a general problem with the English language all over the manuscript. Please revise the manuscript for grammatical and syntax errors as well as missing articles (a lot are missing).

Author Response

Thanks very much for taking your time to review our manuscript. I really appreciate all your comments and suggestions. We have carefully considered the suggestion of Reviewers and make some changes. We have tried our best to improve and made some changes in the manuscript. The red parts have been revised according to your comments. Revision notes, point-to-point, are given as follows:

Review 1:

1. In the manuscript, the authors wrote: “However, the noble metals because of high price, scarcity and inferior durability have limited their further wide practical applications.”Then, they propose the use of transition metal oxides as an alternative. However, there are several environmental issues associated with Co.

Yes, we have heard of some studies that show cobalt could cause some environmental pollution to the environment. However, reports have shown that compounds containing cobalt have good performance in OER.

For ordinary consumers, ensuring battery performance while also reducing purchasing costs is the best. We believe that cobalt free batteries, which have advantages in cost, safety and performance are indeed a direction for the development of the battery in future.

2. The purpose of the article is not clearly addressed in the manuscript. The materials are not new, nor are the synthesis method. The authors should better explain the novelty of the work in the introduction.

I am so sorry about that, the novelty of this work was to study in terms of the various temperatures influence on the structure, morphology and surface properties of FeCo2O4/NF, these features have been correlated to the electrochemical behavior for the OER.

3. In the introduction, the authors wrote: “Zhao reported FeCo2O4@FeCo2S4@PPy catalysts through a facile preparation, which exhibited an excellent electrocatalytic performance with overpotential of 98.2 mV for HER at 10 mA cm-2and 280 mV for OER [11].” However,“facile preparation”is abstract. Please clarify what the synthesis method was.

I am so sorry about that, its paper report: “We design novel sandwich-like FeCo2O4@FeCo2S4@PPy catalysts through a facile preparation strategy”. In fact, this “facile preparation” indicated “a three-step electrochemical route”. We have clarified it in our paper.

4. Please indicate the signal amplitude of EIS measurements in the experimental.

Yes, we have added “the signal amplitude of EIS is 5 mV” in the paper of experimental.

5. Figure 1. Data at 160 °C. There is an unrecognizable character in the middle of the pattern. Also, there is an unidentified peal on the left side of the (440) one. Please also indicate the PDF file for the Fe2O3phase.

We examined all the peaks of FeCo2O4 samples, there were two weak peaks of (311) and (440) crystal planes from Fe3O4 (JCPDS no.88-0315). So, it found that amount of Fe3O4 were coexisted in the FeCo2O4 structure for FeCo2O4-140 and FeCo2O4-160 [14].

6. In the results, the authors wrote: “Such interconnection of the nanosheets construct flower-like microspheres with a diameter 2 mm”. From the SEM images, the diameter is much higher than this value. Can the authors clearly indicate in the micrographs this value?

In the paper, we describe the flower-like microspheres with a diameter 2 mm, in fact, some more than it, so we have corrected the diameter about 2~5 mm. 

7. To support the discussion of the morphology differences with temperature (with a subsequent impact on the electrical properties), surface area measurements are needed (e.g., BET analysis).

From the SEM images, amount of nanowires grown aggregately distributed on NF at 100 ºC, these structure could not provide more contact area for OER, which was accordance with the ECSA study. When the temperature increased, nanosheets were grown vertically on the NF to form flower-like structure. As pervious literature reported, nanosheets of catalysts often improve their electrochemical activity, which have a higher specific surface area, consequently increase the contact area between the electrolyte and reactants. When the temperature further increased, FeCo2O4 nanowires were appeared and wrapped FeCo2O4 nanosheets. Further, the growth FeCo2O4 nanowires aggregated together, which also could not facilitate more area for contact between the electrolyte and reactants. So, their electrocatalytic activity decreased. 

8. The XPS analysis should be revised. The background doesn’t seem correct, as it is cutting the data. Please choose carefully the background function as well as its parameters. Also, please provide the envelope curve fitting the experimental data. In Figure 4c) the envelope is missing, neither all the experimental points are fitted.

Yes, we have examined carefully about the XPS curves, and remake all spectra in Fig4. We used Xray photoelectron spectroscopy equipped with a monochromated Al Ka X-ray radiation as the source for excitation and with C1s peak (284.8eV) as a reference. The measurement was performed at pass energy of 20 eV and energy increment of 0.1 eV. The spectra were corrected for the background using the Smart approach.

9. The discussion of Figures 5 and 6 is insufficient. The authors should give a proper scientific explanation for the tendency of the results with increasing temperature. Currently, the authors are just describing the results, which is insufficient for a scientific article. Here, a correlation between microstructure and surface area measurements is needed.

The Figures 5 and 6 was rewritten: The cyclic voltammetry of all the electrodes exhibited a pair of reversible anodic and cathodic redox peaks at about 1.42 and 1.25 V vs RHE, respectively (Fig.5a). These reversible redox peaks maybe attributed to the reversible electron injection (Co2+-OH + OH–↔Co3+-OOH + H+ + e–) for anodic and cathodic. Compared with FeCo2O4/NF-100, the peaks of other samples shown a small negative shift (lower potential) and more current density, indicating that there are better electrochemical activity. In Fig.6 (a), (b), (c) and (d), CVs of FeCo2O4/NF-120 in a non-faradaic region were collected. The plot of the current density against the scan rate had a linear relationship and its slope was Cdl.. The bigger Cdl implied more active sites during the catalytic reaction, consequently promotes highly efficient OER performance.

10. The EIS data must be orthonormal, and the frequency (log10) should be added to the points. Please describe the equivalent circuit used and provide fitting parameters with errors.

Yes, EIS measurements were carried out to further elucidate the electrocatalytic activity of FeCo2O4/NF. The contact resistances (Rs) and charge-transfer resistances (Rct) can be obtained from Nyquist plots of EIS test results fitted by equivalent circuit model (Fig.7). The inset shows the equivalent circuit used for fitting the Nyquist plots, where Rs and Rct are the series and charge-transfer resistances, respectively.

11.Table 1 should also compare with other FeCo2O4 samples from the literature.

Yes, we have changed some literatures from [25] to [27], which were about FeCo2O4 samples.

Reviewer 2 Report

Comments and Suggestions for Authors

In this work, the authors present the synthesis and characterization of multi-metal oxide (FeCo2O4) electrodes as an electrocatalyst for OER in a 1M alkaline medium, with their electrochemical performance. They report an interesting strategy to prepare electrocatalysts for facile oxygen evolution reaction (OER). This work would be useful in relevant fields. I think it can be accepted for publication after some revisions as follows.

1.     The authors need to do literature survey and compare performance of other multi-metal alloy systems. If possible, more theoretical discussion would be helpful to support the novelty of this work (i.e., ACS Catal. 2022, 12, 3821; Appl. Sur. Sci. 2021, vol. 554, 149591 and refs therein).

2.     The authors report reasonably good electrochemical performance of the optimized electrocatalyst. Discuss the validity and reproducibility of the values. The obtained overpotential values are very low (124 mV at 10 mA/cm2). Are these values correct? In addition, have the authors measured overpotential values at higher current densities up to 1 A/cm2 ?

3.      Long-term OER testing could infuence the composition, structure, and performance of the electrocatalyst, which is monitored by Raman spectroscopy, XPS, EDAX, and SEM (i.e., J. Mater. Chem. A, 2022, vol. 10, 20497). If possible, can the authors provide comments on this?

4.     For the OER stability test (Fig. 8), the authors should check the data. Is this chronopotentiometry measurement? (i.e., ACS Applied Energy Materials 2021, vol. 4, 14169; ACS Omega 2019, vol. 4, 3493). Discuss this.

5.     Can the authors provide the real composition of the samples?

6.     There are typos and mistakes in the manuscript. The authors should prepare revision carefully. 

Comments on the Quality of English Language

There are some typos and grammar errors.  

Author Response

Thanks very much for taking your time to review our manuscript. I really appreciate all your comments and suggestions. We have carefully considered the suggestion of Reviewers and make some changes. We have tried our best to improve and made some changes in the manuscript. The red parts have been revised according to your comments. Revision notes, point-to-point, are given as follows:

Review 2:

1. The authors need to do literature survey and compare performance of other multi-metal alloy systems. If possible, more theoretical discussion would be helpful to support the novelty of this work (i.e., ACS Catal. 2022, 12, 3821; Appl. Sur. Sci. 2021, vol. 554, 149591 and refs therein).

In order to support the novelty of the FeCo2O4/NF catalysts, we provided some studies about the multi-metal oxide of nanosheets, indicated they had large specific surface area, which could increase the contact area between the catalyst and reactants, it was more important to improve the OER activity.

2. The authors report reasonably good electrochemical performance of the optimized electrocatalyst. Discuss the validity and reproducibility of the values. The obtained overpotential values are very low (124 mV at 10 mA/cm2). Are these values correct? In addition, have the authors measured overpotential values at higher current densities up to 1 A/cm2 ?

Yes, we have repeated and verified the values of overpotential many times. Qi []report the FeCo2O4/carbon nanofiber with an overpotential of 130 mV at 10 mA cm-2, The introduction of iron in the structure of cobalt spinel creates oxygen vacancies, which should increase the conductivity of the material, what finally entails a higher catalytic activity. We would measure the higher current densities up to 1 A/cm2 in future for further research.

3. Long-term OER testing could influencethe composition, structure, and performance of the electrocatalyst, which is monitored by Raman spectroscopy, XPS, EDAX, and SEM (i.e., J. Mater. Chem. A, 2022, vol. 10, 20497). If possible, can the authors provide comments on this?

We have added the SEM images of the sample FeCo2O4-120 after cycling (Fig. S4), it shows that the morphology does not change significantly. It may due to FeCo2O4 nanosheets grown in situ on NF be able to avoid the falling problem of active material, which caused by a large number of bubbles during OER reactions, and ensure the good catalytic stability of electrocatalyst. 

4. For the OER stability test (Fig. 8), the authors should check the data. Is this chronopotentiometry measurement? (i.e., ACS Applied Energy Materials 2021, vol. 4, 14169; ACS Omega 2019, vol. 4, 3493). Discuss this.

Yes, we have retested the chronopotentiometry measurement. It maintained approximately constant potential of 1.35V at a current density of 10 mA cm-2, indicating good durability in alkaline solution. SEM images of the sample FeCo2O4-120 after cycling (Fig. S4), it shown that the morphology does not change significantly, demonstrating its electrochemical stability, which was due to the existence of stable active sites on the catalyst.  

5. Can the authors provide the real composition of the samples?

Yes, we have shown the XRD the sample FeCo2O4-120. It was found that there are two weak peaks of (311) and (440) crystal planes from Fe3O4 (JCPDS no.88-0315) [14]. So, amount of Fe3O4 coexisted in the FeCo2O4 structure for FeCo2O4-140 and FeCo2O4-160. 

6. There are typos and mistakes in the manuscript. The authors should prepare revision carefully. 

We have corrected the paper carefully, and rewrite it again. Thank you very much.

Reviewer 3 Report

Comments and Suggestions for Authors

The main contribution of this work is the successful synthesis of various hydrothermal temperatures of FeCo2O4/NF nanosheets and the identification of the optimal temperature, 120 ºC, which resulted in the highest OER activity due to the 3D Ni foam providing a good conductive substrate for the FeCo2O4 nanosheets. The synergistic effect between Co and Fe also enhanced the electrocatalytic activity. The strengths of this work include the low overpotential and small tafel slope of the optimized electrode, as well as its long-term stability, which make it a promising candidate for low-cost electrocatalysts in energy conversion and storage.

Specific comments:

1. XPS experimental details are not complete. What was the excitation source, electron emission angle, the size of analyzed area? Where samples sputter-etched prior to analyses? What was the base pressure during analyses? Was charge neutralizer used? All of these aspects are crucial for the correct interpretation of experimental results. Why did the authors not estimate the elemental composition from the area of each photoelectron peak?

2. It is customary to plot XPS spectra with the binding energy decreasing from left to right. Please modify the binding energy axis accordingly. 

Explain the XPS peak fitting procedure in the manuscript - what sort of constraints were applied? What choice of background and peak functions did you make? Refitting.  For fitting details please, as a guidance, see doi.org/10.1116/6.0000377 which can be a hint consistent with JVSTA standards for photoemission. Afterwards, the XPS-related text might need a correction.

3. Further: are there any signals originating from substrate? Survey spectra of FeCo2O4/NF electrode prepared with various hydrothermal temperatures could help to resolve this issue. If there is any signal from substrate that shall be discussed (impact) properly as well.

Figure 3: do Authors have also a chemical mapping by EDX? That would give a hint on uniformity of sample chemical composition. where is there so much C 1s from?

4. SEM /TEM/ EDX experiment description shall be supplemented with full experimental details. Discuss SEM, TEM and HRTEM micrographs at different magnifications in Support Information.

5. Could you elaborate on why you believe FeCo2O4/NF are important?

Author Response

Dear Editor and Dear reviewer:

Thanks very much for taking your time to review our manuscript. I really appreciate all your comments and suggestions. We have carefully considered the suggestion of Reviewers and make some changes. We have tried our best to improve and made some changes in the manuscript. The red parts have been revised according to your comments. Revision notes, point-to-point, are given as follows:

Review 3:

1. XPS experimental details are not complete. What was the excitation source, electron emission angle, the size of analyzed area? Where samples sputter-etched prior to analyses? What was the base pressure during analyses? Was charge neutralizer used? All of these aspects are crucial for the correct interpretation of experimental results. Why did the authors not estimate the elemental composition from the area of each photoelectron peak?

In this paper, we used X-ray photoelectron spectroscopy equipped with a monochromated Al Ka X-ray radiation as the source for excitation and with C1s peak (284.8eV) as a reference. The measurement was performed at pass energy of 20 eV and energy increment of 0.1 eV. The spectra were corrected for the background using the Smart approach. The powder sample is peeled off the FeCo2O4/NF and spread on adhesive tape, pressing it into a thin layer.

2. It is customary to plot XPS spectra with the binding energy decreasing from left to right. Please modify the binding energy axis accordingly. 

Explain the XPS peak fitting procedure in the manuscript - what sort of constraints were applied? What choice of background and peak functions did you make? Refitting.  For fitting details please, as a guidance, see doi.org/10.1116/6.0000377 which can be a hint consistent with JVSTA standards for photoemission. Afterwards, the XPS-related text might need a correction.

We have modified all the XPS spectra with the binding energy decreasing from left to right. The XPS fitting peak was used Avantage software. Firstly, charge correction. Charge compensation is often required when measuring XPS spectra from insulating materials. We corrected the charge of all elements with C1s peak (284.8eV) as a reference. Second, create-baseline. We choose the background using the Smart approach and peak fitting using Add fitting peak. XPS peaks were applied “Gaussian/Lorentzian mix”. We have corrected all the XPS spectra and related text.

3. Further: are there any signals originating from substrate? Survey spectra of FeCo2O4/NF electrode prepared with various hydrothermal temperatures could help to resolve this issue. If there is any signal from substrate that shall be discussed (impact) properly as well.

Figure 3: do Authors have also a chemical mapping by EDX? That would give a hint on uniformity of sample chemical composition. Where is there so much C 1s from?

We prepared XPS powders peeled off the FeCo2O4/NF. We have made chemical mapping of the sample by TEM. In XPS experimental, The C element may come from the adventitious carbon. 

4. SEM /TEM/ EDX experiment description shall be supplemented with full experimental details. Discuss SEM, TEM and HRTEM micrographs at different magnifications in Support Information.

In order to illustrate the structure, we have added SEM、TEM images and EDX in the Support Information. From the enlarged image, it clearly showed a nanosheet structures. Some nanosheets stacked together formed microspheres. When we ultrasonic treatment it, the nanosheets falls off the microsphere. TEM image (Fig. S2) further confirms the existence of a layer of nanosheet. So, from above result, the FeCo2O4/NF material is a nanosheet structure. 

5. Could you elaborate on why you believe FeCo2O4/NF isimportant?

Yes, we think FeCo2O4/NF is important. Ternary metal oxides (TMOs) as the electrode materials have been widely studied. As we known, spinel NiCo2O4, a Ni atom replaced Co, demonstrates higher electrochemical activity and better conductivity than nickel oxide or cobalt oxide. FeCo2O4 structures have attracted widespread attention due to their low cost and environmental friendliness, and high electrochemical activity, which has been regarded as a good electrode material due to two variable valence states of Fe2+ and Fe3+during the electrochemical process. More important, FeCo2O4 in-situ growth could effectively improve the binding strength and affinity between the active material and the matrix. Therefore, FeCo2O4 grown in situ NF can avoid the falling problem of active material caused by a large number of bubbles in OER reactions, and ensure the good catalytic stability of electrocatalyst. So, we believe FeCo2O4/NF is important for OER.

Reviewer 4 Report

Comments and Suggestions for Authors

This study aims to develop low-cost electrocatalysts and is a significant report that mentions the influence of the shape of FeCO2O4 on electrocatalytic activity. However, the description of Figures and Tables and the discussion of each experimental result are insufficient. The report can be accepted with the modification of the following comments.

1.       It is difficult to understand that the material is a nanosheet by looking at the SEM images in Figure 3 and Figure 4.

2.       This report is insufficiently descriptive with respect to Figure 5, Figure 6, and Table 1. The authors should explain the figures and tables without just the right amount. For example, Figure 5a and Figure 6a are not explained.

3.       It is not clear where in Figure the current and resistance values listed in the text are listed. The authors should make these clearly stated.

4.       There is insufficient mention of the reason for the higher activity of FeCo2O4/NF-120. The authors should calculate the specific surface area and compare it for each sample. Also, the proportion of Fe3O4 and FeCo2O4 should be estimated by elemental analysis for the FeCo2O4/NF-140 and FeCo2O4/NF-160 samples.

Comments on the Quality of English Language

The text often contains expressions that are difficult to understand and need to be reconfirmed.

Author Response

Dear Editor and Dear reviewer:

Thanks very much for taking your time to review our manuscript. I really appreciate all your comments and suggestions. We have carefully considered the suggestion of Reviewers and make some changes. We have tried our best to improve and made some changes in the manuscript. The red parts have been revised according to your comments. Revision notes, point-to-point, are given as follows:

Review 4:

1. It is difficult to understand that the material is a nanosheet by looking at the SEM images in Figure 3 and Figure 4.

In order to illustrate the structure, we have added SEM of FeCo2O4/NF. From the enlarged image, it clearly showed a nanosheet structures. Some nanosheets stacked together formed microspheres. When we ultrasonic treatment it, the nanosheets falls off the microsphere. TEM image (Fig. S2) further confirms the existence of a layer of nanosheet. So, from above result, the FeCo2O4/NF material is a nanosheet structure.

2. This report is insufficiently descriptive with respect to Figure 5, Figure 6, and Table 1. The authors should explain the figures and tables without just the right amount. For example, Figure 5a and Figure 6a are not explained.

The cyclic voltammetry of all the electrodes exhibited a pair of reversible anodic and cathodic redox peaks at about 1.42 and 1.25 V vs RHE, respectively (Fig.5a). These reversible redox peaks maybe attributed to the reversible electron injection (Co2+-OH + OH–↔Co3+-OOH + H+ + e–) for anodic and cathodic. Compared with FeCo2O4/NF-100, the peaks of other samples shown a small negative shift (lower potential) and more current density, indicating that there are better electrochemical activity. In Fig.6 (a), (b), (c) and (d), CVs of FeCo2O4/NF-120 in a non-faradaic region were collected. The plot of the current density against the scan rate had a linear relationship and its slope was Cdl.. The bigger Cdl implied more active sites during the catalytic reaction, consequently promotes highly efficient OER performance.

3. It is not clear where in Figure the current and resistance values listed in the text are listed. The authors should make these clearly stated.

Yes, we have added the image of current in supporting information. The OER performance of FeCo2O4 @/NF was estimated by LSV polarization curves and Nyquist plots from EIS. The type of current listed containing 10 mA cm-2 and 50 mA cm-2(Fig.S5). EIS measurements were carried out to further elucidate the electrocatalytic activity of FeCo2O4/NF (Fig.7). The contact resistances (Rs) and charge-transfer resistances (Rct) can be obtained from Nyquist plots of EIS test results fitted by equivalent circuit model. The inset shows the equivalent circuit used for fitting the Nyquist plots, where Rs and Rct were the series and charge-transfer resistances, respectively.

4. There is insufficient mention of the reason for the higher activity of FeCo2O4/NF-120. The authors should calculate the specific surface area and compare it for each sample. Also, the proportion of Fe3O4 and FeCo2O4should be estimated by elemental analysis for the FeCo2O4/NF-140 and FeCo2O4/NF-160 samples.

We added the part of paper to improve the reason for the higher activity of FeCo2O4/NF-120." Combining the above results, the enhancement of OER performance of FeCo2O4/NF-120 was mainly due to the following reason: (1) Flower-like morphology could increase the electrochemical active surface area, which increased the contact with the electrolyte and the more active sites;（2）FeCo2O4 nanosheets grown in situ on NF could improve the conductivity and stability of electrode.″ in addition, there were no Fe3O4 in the FeCo2O4/NF-140 and FeCo2O4/NF-160 samples, so we could not estimate the elemental analysis.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript has been improved.

Author Response

Thanks very much for taking your time to review our manuscript. I really appreciate all your comments and suggestions. We have carefully considered the suggestion of Reviewers and make some changes. We have tried our best to improve and made some changes in the manuscript. The blue parts have been revised.

Reviewer 4 Report

Comments and Suggestions for Authors

The revisions have indeed improved the text's readability, but the credibility of the discussion regarding each experimental result needs to be addressed. We can accept this report with the following modifications:

Many reports asserting the formation of nanosheets in the synthesis of oxide materials use precursors that can be unequivocally controlled at the molecular level, as demonstrated by structural analysis via X-ray diffraction (XRD). These reports often involve the exfoliation of layered compounds containing counter cations to achieve nanosheet structures. To claim that the present compound exhibits nanosheet formation, it is essential to reference processes that are responsible for creating and sustaining nano-level structures in FeCo2O4. Given these considerations, the text should be revised to describe the present compound as having a sheet-like structure.

In the conclusion, the authors assert that "the crystalline form increases the surface area." If this is indeed the case, specific surface areas for each sample should be quantified by obtaining gas adsorption isotherms.

These suggested modifications should help enhance the paper's clarity and credibility.

Comments on the Quality of English Language

Minor corrections are needed, such as typographical errors.

Author Response

Thanks very much for taking your time to review our manuscript. I really appreciate all your comments and suggestions. We have carefully considered the suggestion of Reviewers and make some changes. We have tried our best to improve and made some changes in the manuscript. The blue parts have been revised according to your comments. Revision notes, point-to-point, are given as follows: Many reports asserting the formation of nanosheets in the synthesis of oxide materials use precursors that can be unequivocally controlled at the molecular level, as demonstrated by structural analysis via X-ray diffraction (XRD). These reports often involve the exfoliation of layered compounds containing counter cations to achieve nanosheet structures. To claim that the present compound exhibits nanosheet formation, it is essential to reference processes that are responsible for creating and sustaining nano-level structures in FeCo2O4. Given these considerations, the text should be revised to describe the present compound as having a sheet-like structure. Yes, many reports asserting the formation of nanosheets in the synthesis of oxide materials use precursors that can be unequivocally controlled at the molecular. In previous reports, NH4F was very important for the nanosheet of oxide materials [13,17]. NH4F not only acted as a precipitant, resulting in co-precipitation of Fe2+ and Co2+, but also as additives to control the morphology of metal oxides precursors in reactions. It can generate ammonia under the hydrothermal conditions, which controlled the formation of metal ion-amino complexes followed by release of metal ions in the growth solution. Then ammonium salt ions reacted with the metal ions (released from the complexes) to produce metal oxide precursor nanoparticles. When metal oxide nuclei were formed in the initial crystal growth stage, metal oxide crystals grow up into nanosheets. We have added these descriptions in our paper. The nanosheets structure were also investigated by SEM and TEM. In the conclusion, the authors assert that "the crystalline form increases the surface area." If this is indeed the case, specific surface areas for each sample should be quantified by obtaining gas adsorption isotherms. Yes, the specific surface area of the catalyst plays an important role in catalytic activity. In fact, we know the surface area of sample is synthesized by BET-N2 adsorption with powders. If FeCo2O4/NF sample were measured by BET, FeCo2O4 powders should be peeled off by ultrasonic treatment, which would lead to nanostructure to be aggregated or destroyed. So, we think this test couldn’t really reflect its true surface area. The electrochemical active surface area, which reflects the catalytic active sites of catalyst for electrochemistry activity and provide sufficient channels for charge transfer and material transfer on the electrode surface.We think the magnitude of ECSA is paramount to the assessment of the catalyst’s activity. We also corrected it by ″ provide more electrochemical active area".