Formation of MXene-Derived/NiCoFe-LDH Heterostructures for Supercapacitor Applications
Round 1
Reviewer 1 Report
Reviewer’s comments
Manuscript Number: materials-2081625
Title: Formation of a Three-Dimensional Interconnected NiCoFe–LDH/V2CTx–MXene Network for Supercapacitor Applications
Journal: materials
1. EIS data (Fig. 7e) should be fitted with an equivalent circuit, and the electrochemical parameters should be obtained and discussed.
2. It is unexpected that the composite materials NiCoFe–LDH/V2CTx–MXene shows higher resistance than its counterparts (NiCoFe–LDH and V2CTx–MXene). Deep discussion and strong justification should be provided.
- The correlation between structural/morphological findings and electrochemical performance should be discussed and compared with other related composites such as: Journal of Solid State Electrochemistry, 18 (9) (2014) 2505–2512
4. It is recommended to show the pseudocapacitance and EDLC contribution using Trasatti’s analysis (the following references may help for a better explanation: Chemical Engineering Journal 409, 2021, 128216, Chemical Engineering Journal 419, 2021, 129576.
5. It is recommended to increase the stability test upto 10000 cycles.
Author Response
Response to reviewer
We truly appreciate the reviewer's comments and suggestions. We have revised the manuscript and the major changes made to the reviewer's comments are highlighted in green in the latest draft, while corrections to some grammatical errors, formatting errors, etc. are highlighted in red. Our response to each comment is included below. We hope that we have fully answered all of the reviewers' questions.
- EIS data (Fig. 7e) should be fitted with an equivalent circuit, and the electrochemical parameters should be obtained and discussed.
Response: Thank you for your comments. The equivalent circuit has been added in Fig.7e. The electrochemical parameters are also discussed in the revised manuscript.
- It is unexpected that the composite materials NiCoFe–LDH/V2CTx–MXene shows higher resistance than its counterparts (NiCoFe–LDH and V2CTx–MXene). Deep discussion and strong justification should be provided.
Response: Thank you for your insightful question. Sorry for the mistake. Figure 7 was not correctly labelled. We have corrected these in the revised manuscript.
- The correlation between structural/morphological findings and electrochemical performance should be discussed and compared with other related composites such as: Journal of Solid State Electrochemistry, 18 (9) (2014) 2505–2512
Response: Thank you for your comments. We have compared the electrochemical performance with the recommended reference and cited it in the revised manuscript.
- It is recommended to show the pseudocapacitance and EDLC contribution using Trasatti’s analysis (the following references may help for a better explanation: Chemical Engineering Journal 409, 2021, 128216, Chemical Engineering Journal 419, 2021, 129576.
Response: Thank you for your recommendation. We have carefully read these references and cite them in the revised manuscript.
- It is recommended to increase the stability test upto 10000 cycles.
Response: Thank you for your recommendation. We have increased the stability test to 10000 cycles. The charge/discharge stability at 10 A g-1 shows that the capacitance performance of the double electrode remains at 85.3% after 10000 cycles.
Reviewer 2 Report
Dear Authors,
This research require redo or changing title and perspective.
First, you failed to obtain V2CTx few-single-layers. The MXene formation process was not efficient which can be seen by XRD spectra. You got mixture of V2CTx multilayers with unetched MAX phases, which can be considered as a dead weight material for supercaps application. Yours procedure clearly lacks intercalation with some TBAOH / TMAOH ions followed by sonication etc. Please take a look how singlelayer V2CTx, should look like 10.1016/j.msec.2020.111431. Moreover, it seems your pattern is shifted. It was a Co lamp?
Second, Fig 2a clearly shows multilayered structure. Not delaminated. No nanosheets.
Third, V2CTx are lowest stable MXenes, which decompose toward mixture of vanadium oxides in several hours, just by placing them on desk without argon, fridge and UV protection. In your process, you applied hydrothermal procedure 120 oC for 10h. Whereas, seeing that up to 90 oC is considered as "safe" temperature for Ti3C2Tx samples. The citric acid as antioxidant and stabilizer was smart move. However, it did not work. Please check your XPS. There is no visible V-C bond in V 2p nor C-V bond in C 1s which could prove, that MXenes did not oxidize during the process. Also, the most crucial and highest peak - O 1s spectra is missing. There, we could easilly see the V2O5 content. -> 10.1007/s10562-021-03589-6
So, I would reconsider this work after major revision
I would start from working with properly formed single-layer MXene sample. Not the dirty, contaminated one, you obtained.
If you want to keep the claim, that you have MXene-LDH heterostructures, you have to prove that MXenes do not undergo oxidation during hydrothermal procedure. So, do all the steps without addint Ni, Co, Fe compounds and check SEM / XRD / XPS. Still you are working mostly on multilayers not monolayers.
The easiest way is just to change the title to "Formation of MXene-derived/NiCoFe-LDH heterostructures for supercapacitors". Drop the thing that you work on singlelayers. Admit, the material undergo oxidation during hydrothermal process. Show 0 1s of XPS. Rewrite the characterization part.
Just write that it was your intention to obtain MXene-derived vanadium oxides instead, you tried and as I see you failed - and the story is legit.
The V2O5 micro/nanostructures are pretty good materials for supercaps application. -> 10.1007/s11051-019-4645-8
Also remember that the carbide derived carbon is used as an active material for the electrodes in supercaps. So MXene decomposition is not necessary a bad thing.
Porosimetry and electrochemical performance is good.
If you choose the last option (least laboratory work and the most sensible), please rember to rewrite the introduction to cover the vanadium oxides part.
All the best.
Author Response
We truly appreciate the reviewer's comments and suggestions. We have revised the manuscript and the major changes made to the reviewer's comments are highlighted in green in the latest draft, while corrections to some grammatical errors, formatting errors, etc. are highlighted in red. Our response to each comment is included below. We hope that we have fully answered all of the reviewers' questions.
This research require redo or changing title and perspective.
First, you failed to obtain V2CTx few-single-layers. The MXene formation process was not efficient which can be seen by XRD spectra. You got mixture of V2CTx multilayers with unetched MAX phases, which can be considered as a dead weight material for supercaps application. Yours procedure clearly lacks intercalation with some TBAOH / TMAOH ions followed by sonication etc. Please take a look how singlelayer V2CTx, should look like 10.1016/j.msec.2020.111431. Moreover, it seems your pattern is shifted. It was a Co lamp?
Response: Thank you so much for your insightful question. From the SEM image (Figure 2a) and TEM image (Figure 2b), the obtained V2CTx are few-single-layers. The lamp we used is Cu.
Second, Fig 2a clearly shows multilayered structure. Not delaminated. No nanosheets.
Third, V2CTx are lowest stable MXenes, which decompose toward mixture of vanadium oxides in several hours, just by placing them on desk without argon, fridge and UV protection. In your process, you applied hydrothermal procedure 120 oC for 10h. Whereas, seeing that up to 90 oC is considered as "safe" temperature for Ti3C2Tx samples. The citric acid as antioxidant and stabilizer was smart move. However, it did not work. Please check your XPS. There is no visible V-C bond in V 2p nor C-V bond in C 1s which could prove, that MXenes did not oxidize during the process. Also, the most crucial and highest peak - O 1s spectra is missing. There, we could easilly see the V2O5 content. -> 10.1007/s10562-021-03589-6
So, I would reconsider this work after major revision
I would start from working with properly formed single-layer MXene sample. Not the dirty, contaminated one, you obtained.
If you want to keep the claim, that you have MXene-LDH heterostructures, you have to prove that MXenes do not undergo oxidation during hydrothermal procedure. So, do all the steps without addint Ni, Co, Fe compounds and check SEM / XRD / XPS. Still you are working mostly on multilayers not monolayers.
The easiest way is just to change the title to "Formation of MXene-derived/NiCoFe-LDH heterostructures for supercapacitors". Drop the thing that you work on singlelayers. Admit, the material undergo oxidation during hydrothermal process. Show 0 1s of XPS. Rewrite the characterization part.
Just write that it was your intention to obtain MXene-derived vanadium oxides instead, you tried and as I see you failed - and the story is legit.
The V2O5 micro/nanostructures are pretty good materials for supercaps application. -> 10.1007/s11051-019-4645-8
Also remember that the carbide derived carbon is used as an active material for the electrodes in supercaps. So MXene decomposition is not necessary a bad thing.
Porosimetry and electrochemical performance is good.
If you choose the last option (least laboratory work and the most sensible), please rember to rewrite the introduction to cover the vanadium oxides part.
Response: Thank you so much for your insightful question. As you suggested, we change the title into “Formation of MXene-derived/NiCoFe-LDH heterostructures for supercapacitors”. As you can see, the element V was partially oxidized into V2O5. V2O5 is also a good material for the energy storage. That means the oxidation is good for the electrochemical performance. We modified the corresponding content of the V2CTx in the revised manuscript.
Reviewer 3 Report
Comments to the Authors:
In this paper, authors report the “Formation of a Three-Dimensional Interconnected NiCoFe-2
LDH/V2CTx–MXene Network for Supercapacitor Applications”. It is significant to point the core issues of energy storage current directly. The experimental design is novel, the experimental data is complete, and the writing is standard. In summary, it can meet the publication requirements. Before the publication however some changes in text are necessary.
Comments:
1. The purity of chemicals should be included in the experimental part. How the authors optimized the experimental parameters for MXene synthesis?
2. How about the morphology after cycles? Whether the morphology collapsed after cycling? Please compare the morphology of electrode material before and after cycles, if possible.
3. The necessary comparison to the recent electrode material was suggested to highlight the advantages of this work.
4. The cycling performance of electrode materials at different current densities should be measured, in order to analyze their electrochemical properties more comprehensive.
5. Author stated that “The potential window was set in the range of 0 to 0.4 V to avoid water electrolysis during charging, for GCD analysis, and 0 to 0.5 V window applied for CV analysis, why? In CV there is no water electrolysis during charging above the 0.45 V?
6. The author needs to provide the EIS data for bare samples with the integrated sample, also before and after cycling stability test.
7. Is there any changes observed in NiCoFe–LDH/V2CTx–MXene material after the cyclic stability (XRD, SEM, TEM, XPS)?
8. What is the nature of the interaction between MXene, LDH, is it only adsorption?
9. Why the authors used 7000 cycles for the three-electrode system and 3000 for the device should be explained. And what happened to the maintenance of cycle stability after 6000 and 3000 cycles?
Author Response
We truly appreciate the reviewer's comments and suggestions. We have revised the manuscript and the major changes made to the reviewer's comments are highlighted in green in the latest draft, while corrections to some grammatical errors, formatting errors, etc. are highlighted in red. Our response to each comment is included below. We hope that we have fully answered all of the reviewers' questions.
- The purity of chemicals should be included in the experimental part. How the authors optimized the experimental parameters for MXene synthesis?
Response: Thank you for your comments. The purity of the chemicals has been added in the experimental section. We optimized the experimental parameters by control the hydrothermal reaction time, the ratio of the metal ions in the layered double hydroxides, the amount of urea used in the hydrothermal reaction. It is a complicated process, but not gives important results. Therefore, we do not include these data in the manuscript.
- How about the morphology after cycles? Whether the morphology collapsed after cycling? Please compare the morphology of electrode material before and after cycles, if possible.
Response: Thank you for your insightful question. The charge/discharge stability at 10 A g-1 shows that the capacitance performance of the NiCoFe–LDH/V2CTx–MXene//AC device remains at 85.3% after 10000 cycles. We do check the morphology collapsed after cycling. But from the cycling performance of the device,
We believe the electrode shows very good stability.
- The necessary comparison to the recent electrode material was suggested to highlight the advantages of this work.
Response: Thank you for your suggestion. The performance of the recent electrode materials have been compared with this work in Table 2 and Figure 9c.
- The cycling performance of electrode materials at different current densities should be measured, in order to analyze their electrochemical properties more comprehensive.
Response: Thank you for your suggestion. The charging and discharging performance at the different current densities have been discussed in the manuscript. Therefore, we do not check them in the cycling performance.
- Author stated that “The potential window was set in the range of 0 to 0.4 V to avoid water electrolysis during charging, for GCD analysis, and 0 to 0.5 V window applied for CV analysis, why? In CV there is no water electrolysis during charging above the 0.45 V?
Response: Thank you for your insightful question. For the GCD analysis, the potential range of 0 to 0.4 V was used. We tried to set a higher potential to 0.5 V. But the electrode cannot be charged to 0.5 V. Therefore, we have to select the potential range of 0 to 0.4 V.
- The author needs to provide the EIS data for bare samples with the integrated sample, also before and after cycling stability test.
Response: Thank you for your suggestion. The EIS data for the bare samples and the integrated samples are provided in Figure 7e.
- Is there any changes observed in NiCoFe–LDH/V2CTx–MXene material after the cyclic stability (XRD, SEM, TEM, XPS)?
Response: Thank you for your insightful question. The charge/discharge stability at 10 A g-1 shows that the capacitance performance of the NiCoFe–LDH/V2CTx–MXene//AC device remains at 85.3% after 10000 cycles. We do check the morphology collapsed after cycling. But from the cycling performance of the device,
We believe the electrode shows very good stability.
- What is the nature of the interaction between MXene, LDH, is it only adsorption?
Response: Thank you for your insightful question. The interaction between the MXene and LDH is electrostatic attraction. The MXene is positively charged, while the LDH is negatively charged.
- Why the authors used 7000 cycles for the three-electrode system and 3000 for the device should be explained. And what happened to the maintenance of cycle stability after 6000 and 3000 cycles?
Response: Thank you for your insightful question. After 6000 cycles for the three-electrode system, the capacitance retention of the electrode is lower than 85%. Therefore, we stopped the further test, The same thing for the device.
Round 2
Reviewer 1 Report
The authors have addressed most of the comments in the revised version. The current version could be accepted for publication.
Author Response
Thank you so much for your comments. We have carefully checked the manuscript and corrected the typos in the manuscript.
Reviewer 3 Report
The reviewer is satisfied with the author's response. However, some minor errors were still found in the revised manuscript, so it should be carefully revised before publication in the material.
Example. Fig.8(b) and (c): 2mV/S and 0.4 mV/S should be changed to 2 mV/s
Author Response
Thank you so much for your comments. We have carefully checked the manuscript and corrected the typos.
