First-Principles Study on Desolvation and Capacitive Performance of Bispyrrolidinium Cations in Pristine/Oxygen-Functionalized Bilayer Graphene Flat Pores
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript under review presents a detailed theoretical investigation into the desolvation behavior of bispyrrolidinium cation complexes within functionalized graphene-based electrode materials for supercapacitors. Using DFTB+ calculations, authors explore how different oxygen-containing surface functional groups affect the desolvation process, intercalation energetics, and overall capacitance performance. The study is motivated by a central challenge in high-performance supercapacitors: the limited ion storage efficiency arising from incomplete or energetically unfavorable desolvation of electrolyte cations. The work aims to offer a microscopic understanding of how surface chemistry influences ion–electrode interactions and energy storage capability.
The computational methodology, including geometric optimization, energy profile evaluation, and electronic structure analysis, appears rigorous enough and appropriate for the stated goals. The identification of critical pore diameters for full desolvation and the distinct roles of hydroxyl and carbonyl groups improves our understanding of desolvation mechanisms and functionalization effects in graphene systems.
There are few suggestions for the manuscript:
- Eint1 - Eint5 enrgies numbering is not intuitive, especially for the figures 2 - 7. One might introduce more meaningful names for energies.
- In fig. 9 the "electronics/eV" unit is confusing. Should it be electrons or number of states?In the corresponding description, the phrases like "the peak value at the
Fermi level decreases from 16.96 eV to 11.22 eV" looks strange.
Besides, the changes at the Fermi level are quite minor. How confident authors are that this difference is not caused by calculation error? - There are few misprints, like "indicating" usage in paragraph 3.2.1.
- The first phrase in conclusion "In this experiment, the researchers employed..." is suspicious. Is it not written by researchers? AI?
After correcting these issues, the manuscript could be published.
Author Response
Response to Reviewer Comments
Dear Reviewer,
Thank you very much for taking the time to review this manuscript. I sincerely appreciate all your comments and suggestions! Your rigorous attitude towards academic papers is a role model for me. Below are my itemized responses, and the revised content has been included in the resubmitted files.
Point 1: Eint1 - Eint5 enrgies numbering is not intuitive, especially for the figures 2 - 7. One might introduce more meaningful names for energies.
- Insight into the Desolvation of Quaternary Ammonium Cation with Acetonitrile as a Solvent in Hydroxyl-Flat Pores: A First-Principles Calculation.
https://doi.org/10.3390/ma16103858
- Insight into the Desolvation of Organic Electrolyte Cations with Propylene Carbonate as a Solvent in Flat Pores: A First-Principles Calculation
https://doi.org/ 10.3390/coatings13081384
- First-Principles Calculation of the Desolvation Effect of Functionalized Carbon Nanotubes
https://doi.org/10.3390/coatings15101190
Point 2: In fig. 9 the "electronics/eV" unit is confusing. Should it be electrons or number of states?In the corresponding description, the phrases like "the peak value at the Fermi level decreases from 16.96 eV to 11.22 eV" looks strange.Besides, the changes at the Fermi level are quite minor. How confident authors are that this difference is not caused by calculation error?
Response 2:
Thank you for your valuable comments on our manuscript. We highly appreciate your careful review and insightful questions, which help improve the accuracy and rigor of our work. Below is a detailed response to each of your concerns:
- Clarification on the "electronics/eV" Unit in Figure 9
The "electronics/eV" label in Figure 9 is indeed an error in unit notation. The correct unit for the y-axis of Density of States (DOS) plots should be "states/eV" (number of states per electron volt), which quantifies the number of available electronic states within a given energy interval.
This mistake arose from a typographical error during figure preparation. We have corrected the unit to "states/eV" in the revised figure and verified consistency with standard DOS plot conventions in computational materials science.
- Correction on the Description of Fermi Level Peak Values
The phrase "the peak value at the Fermi level decreases from 16.96 eV to 11.22 eV" is inaccurate, as the peak value of DOS is a dimensionless quantity (number of states) rather than an energy (eV). The correct description should refer to the DOS intensity (states/eV) at the Fermi level (Ep = 0 eV).
For clarity, we revise the relevant description as follows (taking CFP as an example):
Original: "In the AA stacking, the peak value at the Fermi level decreases from 16.96 eV to 11.22 eV"
Revised: "In the AA stacking, the DOS intensity at the Fermi level decreases from 16.96 states/eV to 11.22 states/eV"
We have systematically corrected all similar descriptions in the manuscript to avoid confusion between energy (eV, x-axis) and DOS intensity (states/eV, y-axis).
- Confidence in the DOS Difference: Exclusion of Calculation Error
We confirm that the observed differences in DOS intensity at the Fermi level are not caused by calculation errors, supported by the following evidence:
Convergence verification: We strictly tested the convergence of k-point grids (1×1×1, 2×2×1, 3×3×1) and geometric optimization parameters (energy error < 0.05 kcal/mol, force convergence < 0.5 kcal/mol/Å). The 1×1×1 k-point grid adopted in the study yielded consistent DOS results with higher k-point densities (variation < 2%), confirming convergence.
Reproducibility: The DOS calculations for each system (FP, HFP, AFP, CFP) were repeated three times with identical parameters, and the results showed negligible deviations (standard deviation < 0.3 states/eV) in Fermi level intensity.
Physical rationality: The trends in DOS changes align with charge transfer analysis (Bader charge, Table 2). For example, SBP⁺ transfers more electrons to HFP/AFP (≈0.73–0.76 e) than to CFP (≈0.67 e), leading to higher DOS intensity at the Fermi level for HFP/AFP (enhanced conductivity) and lower intensity for CFP (reduced conductivity). This consistency between electronic structure and charge transfer results validates the reliability of the DOS differences.
While the absolute changes in DOS intensity (e.g., CFP: 16.96 → 11.22 states/eV) appear moderate, they are statistically significant and physically meaningful, reflecting the modulation of functional groups on the electrode’s electronic conductivity.
Follow-Up Action
We have revised Figure 9’s unit label and the corresponding text descriptions to eliminate inaccuracies. Additionally, we will add a brief note in the Supplementary Materials to document the convergence test results of DOS calculations, further supporting the reliability of our findings.
Point 3: There are few misprints, like "indicating" usage in paragraph 3.2.1.
Point 4: The first phrase in conclusion "In this experiment, the researchers employed..." is suspicious. Is it not written by researchers? AI?
Response 4:
The opening phrase "In this experiment, the researchers employed..." is an inappropriate expression caused by careless writing during the draft stage. The manuscript is an original work completed independently by the authors, without the involvement of AI tools in content generation.
We have revised the sentence to: “In this study, we employed first-principles calculations to investigate the desolvation behavior and relative capacitance of bispyrrolidinium cation complexes in AA-stacked, AB-stacked pristine flat pores, and oxygen-functionalized flat pores. Additionally, we analyzed the density of states (DOS) and charge density difference of desolvated SBP⁺ embedded in functionalized flat pores.” We hope this revision meets your satisfaction.
Supplementary explanation: All authors confirm that the research design, calculation simulations, data analysis, and manuscript writing are original work, and we have attached a statement of originality to the revised manuscript.
Finally, I have polished the professional English of my manuscript, and I would like to once again express my gratitude for your careful guidance and assistance. Your rigorous and meticulous attitude towards scientific research is a goal I strive to achieve; under your guidance, I have also made steady progress. Wish you all the best!
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThis manuscript presents a systematic first-principles study on the desolvation mechanism of bispyrrolidinium cation complexes () in pristine and oxygen-functionalized bilayer graphene flat pores. The authors investigate the effects of hydroxyl (), carbonyl (), and aldehyde () functional groups, as well as AA and AB stacking configurations, on critical desolvation pore diameters, intercalation energy, relative capacitance, and electronic properties (DOS and charge transfer). The topic is highly relevant to optimizing supercapacitor electrode materials, and the use of DFTB+ for large-scale systems is appropriate. The findings are novel and provide explicit theoretical guidance.
The study is generally well-executed, and the conclusions are clearly supported by the reaction energy calculations and subsequent electronic analyses. The manuscript structure follows a logical flow, and the results are presented clearly. However, some aspects of the presentation, analysis, interpretation, and clarity need refinement before publication.
- The Abstract lists the critical diameters as (FP), (HFP), (AFP), and (CFP). The Conclusion section repeats these values. However, the detailed Results and Discussion sections for AA/AB stacking report different critical sizes for the functionalized pores based on the different stacking. For instance, the complete desolvation size for HFP is (AA-stacking) and (AB-stacking). The conclusion seems to be an averaged or minimum/maximum value: "the pore diameter required for the complete desolvation of in the hydroxyl-functionalized flat pores of bilayer graphene is ". Please clarify how the single value for each functional group in the Abstract and Conclusion (e.g., for HFP) was determined from the individual AA and AB stacking results. The text must explicitly state if the listed value is the average, the smallest one across both stackings, or another determined value. It would be clearer to present the range (e.g., HFP: to ) or to present the separate values in the abstract.
- The Abstract states that embedding improved the conductivity of HFP and AFP but decreased that of CFP. The detailed DOS analysis in Section 3.4 for AFP states that the peak value at the Fermi level increases in both AA and AB stackings, which "suggests a slight improvement in the electrical conductivity". However, the Conclusion states that embedded in and shows enhanced conductivity, but when embedded in and , it shows decreased conductivity. This is a direct contradiction regarding AFP. Please correct the statement in the Conclusion section (Section 4) to accurately reflect the findings in the Abstract and Section 3.4. It appears the mention of in the "decreased conductivity" part of the conclusion is an error and should be removed, as the DOS analysis clearly indicates an increase in the peak value for .
- The paper compares capacitance using a "relative capacitance" ratio based on . The relationship is . The comparison is done using the ratio of between and systems. To improve clarity, please explicitly define the "Relative Capacitance" formula in the text. Is it ? Clarifying this ratio will help the reader immediately understand how the values (e.g., for HFP) correspond to enhanced capacitance.
- In the Abstract, "bipyrrolidinium cations" should be corrected to bispyrrolidinium cations for consistency with the main text's acronym . In the Abstract, "FP" and "OFP" are defined, but the definitions of , , and are a bit fragmented. For clarity, consider adding the full names (e.g., hydroxyl-functionalized (HFP), carbonyl-functionalized (CFP), and aldehyde-functionalized (AFP) pores) to the same sentence where they are first mentioned.
- The term "Bispyrrolidine cations" is used in the text to denote . The full name of is bispyrrolidinium cation. The term should be made consistent throughout the entire manuscript to for and cation complexes for .
The manuscript should be accepted after the authors address the comments, particularly the inconsistency in the critical diameter reporting and the error in the DOS interpretation for AFP in the Conclusion, as well as the minor editorial points.
Author Response
Dear Experts, please refer to the Word document for the specific response content.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors should address the following comments for further improvement to the manuscript.
- It would be better if the authors simplified the abstract since its very lengthy structure makes it boring to read.
- Why the authors chose DFTB+ calculation for this study when there are other theoretical model calculations available in the literature.
- The introduction could have included a schematic diagram about the present study for the readers' understanding.
- The authors could have included a schematic diagram in section 3. 1. Reaction principle for readers' understanding.
- In the introduction, the research problem and existing gaps in the present study are not clearly defined.
- It would be better if the authors discussed the significance of each of the stacked hydroxyl flat pores from Figure 1.
- What could be the reason acetonitrile is used as a solvent for employing this study, when there are solvents available in the labs?
- For simplification and for readers’ understanding, the authors could have made a table for the section “3.2. Desolvation of SBP+ Complexes”.
- According to Figure 8, the authors could have discussed why interlayer spacing increases with increasing distance from SBP+ to FP and OFP and why interlayer spacing decreases with increasing relative capacitance. It should be discussed.
- The authors could have briefly discussed future perspectives and opportunities for the present study.
Author Response
Dear Experts, please refer to the Word document for the specific response content.
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for AuthorsThe submitted manuscript by Fudong Liu et al to Coatings/MDPI presents a systematic investigation of the desolvation mechanism of bispyrrolidinium cation complexes using first-principles calculations based on density functional tight-binding (DFTB). The study explores acetonitrile as a solvent within pristine bilayer graphene flat pores (FP) and oxygen-containing functionalized pores—specifically carbonyl- and aldehyde-functionalized—under AA and AB stacking configurations. The analysis of pore structure and surface functionalization offers promising insights into improving desolvation efficiency and capacitance performance in graphene-based supercapacitor electrodes.
Graphene’s exceptional physical properties and its non-magnetic behavior, resulting from its 2p electron system, make it an ideal electrode material for electric double-layer capacitors (EDLCs). While the manuscript is generally well-written, several revisions are necessary to strengthen its technical depth and relevance to supercapacitor applications.
The specific points are given below.
- The title and abstract emphasize supercapacitor applications, yet the Results and Discussion section lacks sufficient analysis related to capacitance behavior. Please expand this section to include a detailed discussion of the parameters that influence the capacitance of graphene-based materials.
- Table 1 refers to “relative capacitance parameters,” but the actual parameters are not clearly defined. Please specify:
- What these parameters represent (e.g., surface area, pore volume, interlayer spacing).
- The relative capacitance values for each stacking configuration under varying interlayer spacing.
- Manickam Minakshi and his group have made substantial contributions to EDLC research. Please incorporate their works and discuss their findings, particularly regarding:
- Double-layer thickness
- Capacitance calculation methodologies
- Influence of electrolyte composition and pore geometry
- The manuscript should address how porosity and surface area impact ion transport and capacitance. Consider including quantitative comparisons or modeling insights to support this.
- The distinction between Figures 2 and 3 is unclear. Please revise the captions and in-text references to clearly explain:
- What does each figure represent?
- How do they differ in terms of structure, functionalization, or simulation results?
- The conclusion could be a bit more effective.
Author Response
Dear Experts, please refer to the Word document for the specific response content.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript has been elaborated thoroughly. Now it deserves publishing.
Author Response
Thank you very much for your recognition, and wish you all the best.
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors responded to the reviewer's comments are satisfactory.
Author Response
Thank you very much for your recognition, and wish you all the best.
Reviewer 4 Report
Comments and Suggestions for AuthorsI have read the author's responses; it appears that they have not taken the utmost care to enhance the quality of their work. Please pay some attention to the queries and submit the next version of the manuscript.
Author Response
Please refer to the reply in the document. Thank you.
Author Response File: Author Response.pdf
