Synthesis of Carbon-Supported MnO₂ Nanocomposites for Supercapacitors Application

Round 1

Reviewer 1 Report

Authors reported capacitance performance MnO2/C. this work is interesting. However, authors need to improve the manuscript Quality.

1.The author needs to check the very blurry SEM image. I think this is because of the high concentration of pt/Au coating on the SEM stub.

2. XPS of C1 should be provided
3. In Figure 6a, b checks the y-axis, it should be at potential (V) and F g-1.
4. Introduction part please highlight importance of carbon and compare results with other materials..Please add these article in revision: (i) Journal of Solid State Chemistry 262, 106-111, (ii) nternational Journal of Hydrogen Energy 45 (57), 32789-32796; (iii) Materials Research Bulletin 107, 446-455; (iv) ournal of Materials Science: Materials in Electronics 29 (18), 15699-15707
5. Authors need to provide the XRD of both samples.
6. There are several typo and format errors.. please revise
7. If possible Add FTIR and BET analysis for better understanding of structural features.

Comments for author File: Comments.pdf

Author Response

Response for Reviewer 1

The authors thank the Reviewer for valuable comments. The manuscript was thoroughly revised according to the Reviewer's suggestions. New figures and some references were added and discussed in the revised version of the manuscript.

Reviewer: The author needs to check the very blurry SEM image. I think this is because of the high concentration of pt/Au coating on the SEM stub.

Authors: SEM images were checked.

Reviewer: XPS of C1 should be provided

Authors: XPS of C1 and survey spectra were added and discussed in the revised version of the manuscript.

Reviewer: In Figure 6a, b checks the y-axis, it should be at potential (V) and F g-1.

Authors: It was checked.

Reviewer: Introduction part please highlight importance of carbon and compare results with other materials. Please add these article in revision: (i) Journal of Solid State Chemistry 262, 106-111, (ii) International Journal of Hydrogen Energy 45 (57), 32789-32796; (iii) Materials Research Bulletin 107, 446-455; (iv) Journal of Materials Science: Materials in Electronics 29 (18), 15699-15707

Authors: The Introduction part was revised and additional references were added.

Reviewer: Authors need to provide the XRD of both samples.

Authors: The XRD patterns of both samples were added and discussed in the revised version of the manuscript.

Reviewer: There are several typo and format errors.. please revise

Authors: It was checked.

Reviewer: If possible Add FTIR and BET analysis for better understanding of structural features.

Authors: We apologize, but at this moment, we can’t do these analyses.

 

Author Response File: Author Response.docx

Reviewer 2 Report

This is a description of the synthesis and characterisation of carbon-supported MnO_x nanocomposites, including the characterisation of charging behaviour (with specific capacitance derived) by cyclic voltammetry and galvanostatic measurements, electron microscopies and XPS. The work comes across as being very competently done (except for the XPS - see below; and I add that I am not en expert in cyclic voltammetry), so the results are sound. I was then expecting a deeper discussion, an attempt to relate the observable microstructure variations visible in electron microscopy and the CV curves to the specific capacitance variations, which would have made the paper much more interesting, and would probably be more useful for other prectitioners in the field. But the results seem sound and can of course be reported as they are. I just think that there is a deeper more interesting story in these data that has not been brought out by the authors.

However, I am not conviced by the XPS analysis, which is, as reported, not able to support the sole conclusion, namely that the materials contain MnO₂. I strongly recommend that the authors consult these sources:

Greczynski, G.; Hultman, L. X-Ray Photoelectron Spectroscopy: Towards Reliable Binding Energy Referencing. Prog. Mater. Sci. 2020, 107, 100591. https://doi.org/10.1016/j.pmatsci.2019.100591.

Greczynski, G.; Hultman, L. Compromising Science by Ignorant Instrument Calibration—Need to Revisit Half a Century of Published XPS Data. Angew. Chemie - Int. Ed. 2020, 59 (13), 5002–5006. https://doi.org/10.1002/anie.201916000.

First, we need to see the C 1s data used for calibrating the BE scale, with an indication how exactly the BE scale was calibrated. Second, the survey data are mentioned, but not shown - so it is impossible to assess the claim that no other elements were detected. Third, the fits used to assign the Mn 2p spectra to MnO₂ are not fits to the data. The compnent lines shown in the plots are not explaining the experimental data - which are much broader and clearly there are several Mn components - not a pure MnO₂ phase. There are also signficant differences in the XPS of S1 ans S2. This probably tells us something interesting and important about the chemical and phase composition of the MnOx phases in thes materials, and the authors may be able to relate this information to the fundamentally different microstructure evident in the electron microscopy results, and then to the capacitance differences. That would then constitute a really nice paper.

This collection of up to date XPS tutorial papers will also be useful:

Reproducibility Challenges and Solutions with a Focus on XPS

https://avs.scitation.org/toc/jva/collection/10.1116/jva.2020.REPROD2020.issue-1

 

Author Response

Response for Reviewer 2

The authors thank the Reviewer for valuable comments. The manuscript was thoroughly revised according to the Reviewer's suggestions. New figures and some references were added and discussed in the revised version of the manuscript.

Reviewer: This is a description of the synthesis and characterisation of carbon-supported MnO_x nanocomposites, including the characterisation of charging behaviour (with specific capacitance derived) by cyclic voltammetry and galvanostatic measurements, electron microscopies and XPS. The work comes across as being very competently done (except for the XPS - see below; and I add that I am not en expert in cyclic voltammetry), so the results are sound. I was then expecting a deeper discussion, an attempt to relate the observable microstructure variations visible in electron microscopy and the CV curves to the specific capacitance variations, which would have made the paper much more interesting, and would probably be more useful for other prectitioners in the field. But the results seem sound and can of course be reported as they are. I just think that there is a deeper more interesting story in these data that has not been brought out by the authors.

However, I am not conviced by the XPS analysis, which is, as reported, not able to support the sole conclusion, namely that the materials contain MnO₂. I strongly recommend that the authors consult these sources:

Greczynski, G.; Hultman, L. X-Ray Photoelectron Spectroscopy: Towards Reliable Binding Energy Referencing. Prog. Mater. Sci. 2020, 107, 100591. https://doi.org/10.1016/j.pmatsci.2019.100591.

Greczynski, G.; Hultman, L. Compromising Science by Ignorant Instrument Calibration—Need to Revisit Half a Century of Published XPS Data. Angew. Chemie - Int. Ed. 2020, 59 (13), 5002–5006. https://doi.org/10.1002/anie.201916000.

Authors: The Authors thank the Reviewer for valuable XPS literature sources.

Reviewer: First, we need to see the C 1s data used for calibrating the BE scale, with an indication how exactly the BE scale was calibrated.

Authors: The analyzer work function was determined, assuming the binding energy or the Au4f7/2  peak to be 84.0 eV. On insulating samples, a high-resolution spectrum was taken of the adventitious hydrocarbon on the sample's surface to use as a reference for charge correction. The generally accepted binding energy for adventitious carbon is 284.8 eV. There is no simple, fundamentally correct method to precisely adjust peak positions for insulating materials. [Donald R. Baer, Kateryna Artyushkova,Hagai Cohen, Christopher D. Easton, Mark Engelhard,Thomas R. Gengenbach, Grzegorz Greczynski, Paul Mack, David J. Morgan, and Adam Roberts. XPS guide: Charge neutralization and binding energy referencing for insulating samples. J. Vac. Sci. Technol. A 38, 031204 (2020); doi: 10.1116/6.0000057]. For XPS analysis, powder samples were homogeneously dusted onto adhesive tape. A broad scan survey spectrum should be obtained first to identify the elements present.

All peaks of the XPS spectra presented here were fitted using 70:30 Gaussian:Lorentzian peak shape, and all the Mn2p multiplet peaks were assigned the same full width at half maximum (FWHM). Nonetheless, the peak shapes (with the exception of the FWHM) for Mn3p are insensitive to changes in Mn bonding environments.[ E.S. Ilton et al. / Applied Surface Science 366 (2016) 475–485].

Reviewer: Second, the survey data are mentioned, but not shown - so it is impossible to assess the claim that no other elements were detected. Third, the fits used to assign the Mn 2p spectra to MnO₂ are not fits to the data. The compnent lines shown in the plots are not explaining the experimental data - which are much broader and clearly there are several Mn components - not a pure MnO₂ phase. There are also significant differences in the XPS of S1 and S2. This probably tells us something interesting and important about the chemical and phase composition of the MnOx phases in these materials, and the authors may be able to relate this information to the fundamentally different microstructure evident in the electron microscopy results, and then to the capacitance differences. That would then constitute a really nice paper.

This collection of up to date XPS tutorial papers will also be useful:

Reproducibility Challenges and Solutions with a Focus on XPS

Authors: The additional survey XPS spectra and XPS spectra of C1s were added and discussed in the revised version of the manuscript.

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Authors addressed all concerns and improved the quality of the manuscript.

Author Response

Response to Reviewer 1

Reviewer: Authors addressed all concerns and improved the quality of the manuscript.

Authors: The authors thank Reviewer.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors have significantly clarified the XPS part of the work, but some important questions still remain. First, I do not find a clear description of the binding energy scale calibration. None of the components in the C 1s fits line up with the stated 284.8 eV. The main component for S1 is even at 284.1 eV - that is extremely unlikely, it should be close to 284.8 eV. So the BE scale needs to be calibrated somehow. The quoted statement that 'There is no simple, fundamentally correct method to precisely adjust peak positions for insulating materials' does not mean that one should not do any calibration of the BE scale - it means that there are alternative ways to do it, and one needs to be very clear about what one does to calibrate the energy scale. Without this clarity the reported BEs for Mn2p are meaningless - as far as I can see, the BE scale in for Mn 2p of S1 probably needs to be adjusted by almost an eV.

Re. the survey spectra: for S1 there is a large amount of what I believe is potassium visible in the spectra (peaks at about 18 eV, 35 eV, 293 eV and 380 eV). The author's are not analysing their XPS data critically enough. The statement 'points to the absence of any other contaminants' is not appropriate. Could K influence the behaviour of the materials?

Author Response

Response for Reviewer 2

We are most thankful for the valuable Reviewer’s comments, which we tried to take into account.

Reviewer: The authors have significantly clarified the XPS part of the work, but some important questions still remain. First, I do not find a clear description of the binding energy scale calibration. None of the components in the C 1s fits line up with the stated 284.8 eV. The main component for S1 is even at 284.1 eV - that is extremely unlikely, it should be close to 284.8 eV. So the BE scale needs to be calibrated somehow. The quoted statement that 'There is no simple, fundamentally correct method to precisely adjust peak positions for insulating materials' does not mean that one should not do any calibration of the BE scale - it means that there are alternative ways to do it, and one needs to be very clear about what one does to calibrate the energy scale. Without this clarity the reported BEs for Mn2p are meaningless - as far as I can see, the BE scale in for Mn 2p of S1 probably needs to be adjusted by almost an eV.

Authors: We expanded the experimental details of XPS analysis in the Materials and Methods part in the revised version of the manuscript. For XPS analysis of MnO₂/C powder samples, the Fermi edge was determined to 0 eV. As the graphitic carbon was used as a substrate for the deposition of MnO₂, the obtained MnO₂/C samples were conductive. For XPS analysis, those samples were dusted onto conductive adhesive carbon tape. Binding energies (BE) were measured without the charging effect of powder samples, and the measured carbon peak corresponded to the graphitic carbon sp² hybridization BE – 284.3±0.2 eV (https://srdata.nist.gov/xps/). Moreover, the measured BE for Mn2p in our samples corresponded to BE values in literature XPS databases.

Reviewer: the survey spectra: for S1 there is a large amount of what I believe is potassium visible in the spectra (peaks at about 18 eV, 35 eV, 293 eV and 380 eV). The author's are not analysing their XPS data critically enough. The statement 'points to the absence of any other contaminants' is not appropriate. Could K influence the behaviour of the materials?

Authors: The authors thank the Reviewer for this comment. K was found in both samples, but its amount was low and reached ca. 3.5 at.% for sample S1 and 0.88 at.% for sample S2. It has been determined that very low amounts of K has no effect on the physical structure and available tunnel space of MnO₂ and specific capacitance (C. Wei, C. Xu, B. Li, H. Du , D. Nan, F. Kang, Anomalous effect of K ions on electrochemical capacitance of amorphous MnO₂, J. Power Sources 234 (2013) 1-4). We did not investigate the influence of K. The statement 'points to the absence of any other contaminants' was removed in the revised version of the manuscript.

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

Please see attached file

Comments for author File: Comments.pdf

Author Response

Response for Reviewer 2

We are most thankful for the valuable Reviewer’s comments, which we tried to take into account. We re-checked our XPS analysis data and apologize for the inconvenience as we found some mistakes, especially for Mn2p3 and C 1s high-resolution spectra. The data of XPS analysis were thoroughly revised and discussed in the revised version of the manuscript.

 

Reviewer: I do not think that the Fermi edge method is very reliable (especially since the C 1s data seem to be indicate that the carbon probed in S1 is mostly sp2 and in S2 mostly sp3 – see below - such materials would have different Fermi edges). In in any case it is not documented by the authors, so it is impossible to judge how exactly the authors have arrived at the BE calibration.

There is clearly a significant difference in the BE scales because the main C 1s maximum for S2 is 0.5 eV higher (284.6 eV) than for S1 (284.1 eV), and the overall shape of the C1s spectra is quite different. There is a significant amount of signal in the ranges above 285 eV, suggesting that a significant amount of carbon is not graphitic.

Authors: We re-checked our XPS analysis data and thoroughly revised and discussed.

Reviewer: The information on the 3.5 and 0.88at% K content needs to be included in the paper (by which technique was it determined? EDAX?). I presume this is the content for the overall sample, including the graphite / carbon.

But based on the XPS results I do think this point needs further discussion in the paper. From the intensity of the K 2p (~294 eV) and the Mn 2p emission lines in the survey it is clear that the K:Mn ratio in S1 is nearly 0.5 (assuming tabulated relative sesitivity factors, Mn 2p is emitted approximately with twice the cross section of K 2p). With such high K loadings a very strong effect on the capacitance was observed in the paper by Wei et al. So is it coincidence that the specific capacitance of S1 is so much lower than for S2? In the paper by Wei et al it is shown that the capacitance reduces with K content – the K content here is higher than any of the K contents in that paper. XRD here indicates that there is no long-range crystallinity in the MnOx (or perhaps KyMnOx) phases. The absence of diffraction lines suggests that long range order does not exceed ~10 nm, probably less. The TEM shows that there are particles with diameters of about 10 nm – so this is consistent. What this suggests is that the XPS elemental analysis is for S1 at least a reasonably good representation of the true element composition, because most of the volume of the spherical nanoparticles is probed by XPS. If all the K is inside the nanoparticles then this nanophase is almost certainly not dominated by MnO2. Did the authors to EDAX of the individual particles ? A high K content in the particles in S1 would then also explain why the XPS data seem overall inconsistent and that would favour my scenario (2) for the XPS BE calibration.

Authors: The information on the 3.5 and 0.88 at% K content was included in the revised version of the manuscript. Those K contents were calculated from XPS analysis data for the overall samples. The K:Mn ratio in S1 and S2 samples was 0.18 and 0.11. We apologize, but at this moment, we haven’t the possibility to carry out an EDAX analysis of the individual particles.

One more we thankful the Reviewer for valuable comments and suggestions. We hope that the revised version of our manuscript will be satisfactory for publication.

Author Response File: Author Response.docx