Life Cycle of Fuel Cells: From Raw Materials to End-of-Life Management

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Overall impression:

The manuscript describes mathematical modeling of various factors involved in the life cycle management of fuel cells, including raw material extraction, component production, operation and maintenance, and ultimately decommissioning and recycling. The method effectively highlights which factors can be prioritized to achieve economic viability for fuel cells.

 

Overall, the manuscript presents the model well, supported by solid background information and reasoning. It provides sufficient data analysis and interpretation of the results. With minor revisions, this work may be suitable for publication in Clean Technologies.

 

Specific comments:

1. The abstract should be clearer and more specific about the topic and methodology. It should explicitly state that the manuscript presents mathematical models.
2. Charts and figures should be relabeled for clarity and specificity. For example, Figures 18 and 19 currently share the same title (“Breakdown for the present value (NPC) for the baseline scenario”).
3. All equations should be properly referenced.
4. In the discussion section, when referencing results from figures, include the corresponding figure number in parentheses to guide the reader.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

English quality is good throughout. It can still be further improved for clarity and readability

Author Response

First, I would like to thank you for your thorough review of our paper (cleantechnol-3854961) and helpful comments to improve it.

 

Reviewer 1

Comments to the Authors
Overall impression:

The manuscript describes mathematical modeling of various factors involved in the life cycle management of fuel cells, including raw material extraction, component production, operation and maintenance, and ultimately decommissioning and recycling. The method effectively highlights which factors can be prioritized to achieve economic viability for fuel cells.

Overall, the manuscript presents the model well, supported by solid background information and reasoning. It provides sufficient data analysis and interpretation of the results. With minor revisions, this work may be suitable for publication in Clean Technologies.

Specific comments:

1. The abstract should be clearer and more specific about the topic and methodology. It should explicitly state that the manuscript presents mathematical models.
2. Charts and figures should be relabeled for clarity and specificity. For example, Figures 18 and 19 currently share the same title (“Breakdown for the present value (NPC) for the baseline scenario”).
3. All equations should be properly referenced.
4. In the discussion section, when referencing results from figures, include the corresponding figure number in parentheses to guide the reader.

 

Comments on the Quality of English Language

English quality is good throughout. It can still be further improved for clarity and readability.

 

To Reviewer 1:

            Thank you very much for your review and valuable remarks.

 

1. The abstract should be clearer and more specific about the topic and methodology. It should explicitly state that the manuscript presents mathematical models.

- We thank the reviewer for this valuable suggestion. We have revised the abstract to make the focus and methodology clearer. In the updated version, we explicitly state that the manuscript develops and applies detailed mathematical models of proton exchange membrane fuel cells (PEMFCs). The revised abstract now highlights that the models cover electrochemical processes, mass and energy balances, degradation dynamics, and cost structures, all evaluated through Life Cycle Assessment (LCA) and Levelized Cost of Electricity (LCOE). This clarification ensures that readers immediately recognize the quantitative modeling approach and its contribution to assessing both environmental and economic sustainability across the full life cycle of fuel cells.

2. Charts and figures should be relabeled for clarity and specificity. For example, Figures 18 and 19 currently share the same title (“Breakdown for the present value (NPC) for the baseline scenario”).

- We appreciate the reviewer’s careful observation. The figures have been carefully reviewed and relabeled to avoid duplication and to provide greater clarity and specificity. Specifically, Figures 18 and 19 now carry distinct titles that accurately reflect their respective content: one refers to the cost component breakdown of the baseline scenario, while the other details the sensitivity analysis of NPC under varying assumptions. Similar adjustments have been applied throughout the manuscript to ensure that each chart and figure is uniquely titled and immediately understandable.

3. All equations should be properly referenced.

- We thank the reviewer for pointing this out. We have revised the manuscript to ensure that all equations are properly numbered and consistently referenced in the main text. Each equation now has a unique sequential number, and citations to the equations have been included in the corresponding explanatory paragraphs. This revision improves the readability of the manuscript and makes it easier for readers to follow the mathematical modeling approach.

4. In the discussion section, when referencing results from figures, include the corresponding figure number in parentheses to guide the reader.

- We appreciate this helpful remark. The discussion section has been revised so that all references to graphical results now explicitly include the corresponding figure number in parentheses. This ensures clearer navigation for the reader and strengthens the connection between the discussion text and the visual results presented in the manuscript.

 

Comments on the Quality of English Language

English quality is good throughout. It can still be further improved for clarity and readability.

- We thank the reviewer for the positive evaluation of the language quality. Following the recommendation, the manuscript has undergone an additional round of language editing to further enhance clarity, readability, and overall flow of the text. Minor stylistic and grammatical improvements have been incorporated throughout the manuscript.

 

Thank you very much for your remarks and comments. They were very useful for me to emphasize the main tasks and contributions of the manuscript, and also to focus the readers attention on the new and unique elements.

 

Reviewer 2 Report

Comments and Suggestions for Authors

The authors presented an overview of the life cycle of fuel cells in terms of their economic and environmental aspects. A model was developed and optimized based on two objectives: environmental and economic aspects. The authors may want to address the following comments.

Figure 7 and 8 describe the sensitivity study of H2 price and CAPEX and LCOE. The "teeth" shape curve of Figure 7 is explained as, "The dense “teeth” are from the connection of discrete 314 levels of CAPEX and mean that for each value in this range, multiple scenarios are generated." on Line 314. The authors may want to clarify the meaning of "multiple scenarios". And what kinds of conclusions should we expect from this curve. Do these two curves represent the sensitivity of CAPEX and LCOE to H2 price?

In the conclusion, the authors mention that a sweet spot operating zone at moderate current densities that can balance the tradeoff between electrical efficiency and the losses in the auxiliary systems. The authors may want to elaborate on the sweet spot range, or an explanation quantitively. The authors may also want to add more details on the optimization methodology and numerical simulations, and if any software has been used for the simulations.

The figures and their explanations are expected in a well-organized way. Each figure describes some relationship between different parameters or variables. However, it is unclear how these figures or relationships are correlated to support the conclusions. It seems that each relationship has its own conclusion, and it is difficult to get a big picture on how these conclusions contribute to the final conclusions. The authors may want to strengthen the connections between different parameters, their relationships, and the conclusions.

Author Response

First, I would like to thank you for your thorough review of our paper (cleantechnol-3854961) and helpful comments to improve it.

 

Reviewer 2

Comments to the Authors
The authors presented an overview of the life cycle of fuel cells in terms of their economic and environmental aspects. A model was developed and optimized based on two objectives: environmental and economic aspects. The authors may want to address the following comments.

1. Figure 7 and 8 describe the sensitivity study of H2 price and CAPEX and LCOE. The "teeth" shape curve of Figure 7 is explained as, "The dense “teeth” are from the connection of discrete 314 levels of CAPEX and mean that for each value in this range, multiple scenarios are generated." on Line 314. The authors may want to clarify the meaning of "multiple scenarios". And what kinds of conclusions should we expect from this curve. Do these two curves represent the sensitivity of CAPEX and LCOE to H2 price?
2. In the conclusion, the authors mention that a sweet spot operating zone at moderate current densities that can balance the tradeoff between electrical efficiency and the losses in the auxiliary systems. The authors may want to elaborate on the sweet spot range, or an explanation quantitively. The authors may also want to add more details on the optimization methodology and numerical simulations, and if any software has been used for the simulations.
3. The figures and their explanations are expected in a well-organized way. Each figure describes some relationship between different parameters or variables. However, it is unclear how these figures or relationships are correlated to support the conclusions. It seems that each relationship has its own conclusion, and it is difficult to get a big picture on how these conclusions contribute to the final conclusions. The authors may want to strengthen the connections between different parameters, their relationships, and the conclusions.

 

 To Reviewer 2:

            Thank you very much for your review and valuable remarks.

 

1. Figure 7 and 8 describe the sensitivity study of H2 price and CAPEX and LCOE. The "teeth" shape curve of Figure 7 is explained as, "The dense “teeth” are from the connection of discrete 314 levels of CAPEX and mean that for each value in this range, multiple scenarios are generated." on Line 314. The authors may want to clarify the meaning of "multiple scenarios". And what kinds of conclusions should we expect from this curve. Do these two curves represent the sensitivity of CAPEX and LCOE to H2 price?

- We thank the reviewer for this insightful comment. To improve clarity, we have revised the text accompanying Figures 7 and 8. An additional sentence was added to explain that the term “multiple scenarios” refers to the combination of discrete CAPEX levels with varying H₂ price values, which results in several simulation outcomes for each price point. We also clarified in the figure captions and discussion that the “teeth” shape curve illustrates how CAPEX and LCOE respond to variations in H₂ price, thus representing the sensitivity of these parameters to the hydrogen cost. Furthermore, the figure titles have been revised for greater precision and readability.

2. In the conclusion, the authors mention that a sweet spot operating zone at moderate current densities that can balance the tradeoff between electrical efficiency and the losses in the auxiliary systems. The authors may want to elaborate on the sweet spot range, or an explanation quantitively. The authors may also want to add more details on the optimization methodology and numerical simulations, and if any software has been used for the simulations.

- We thank the reviewer for this constructive suggestion. To address this, we have added a clarifying sentence at the beginning of the Results section, where the operating sweet spot is now quantitatively defined as being in the range of moderate current densities, based on the trade-off between electrical efficiency and auxiliary losses. Additional details on the optimization methodology and numerical simulations. Additionally, the Conclusion section has been revised to rephrase and emphasize the sweet spot range and the role of the simulations.

3. The figures and their explanations are expected in a well-organized way. Each figure describes some relationship between different parameters or variables. However, it is unclear how these figures or relationships are correlated to support the conclusions. It seems that each relationship has its own conclusion, and it is difficult to get a big picture on how these conclusions contribute to the final conclusions. The authors may want to strengthen the connections between different parameters, their relationships, and the conclusions.

- We thank the reviewer for this valuable observation. To improve the coherence of the results presentation, we have added a new introductory paragraph at the beginning of the Results section. This paragraph explains the interconnections among the figures, highlighting how each figure contributes to the overall analysis and supports the final conclusions. The relationships between parameters such as H₂ price, CAPEX, LCOE, efficiency, and auxiliary system losses are now explicitly linked, thereby providing the reader with a clearer “big picture” view. Cross-references were also introduced throughout the discussion to strengthen the logical flow between figures and the concluding remarks.

  

 Thank you very much for your remarks and comments. They were very useful for me to emphasize the main tasks and contributions of the manuscript, and also to focus the readers attention on the new and unique elements. 

Reviewer 3 Report

Comments and Suggestions for Authors

My comments are:

Authors pointed out mostly the cost analysis of the catalysts, however, the fuel cell cost comprises of other components such as membranes, bipolar plates and sometimes metallic bipolar plates. Please also take these factors into consideration.

Authors must support their claims with the sufficient literatures, entire introduction of the paper doesn’t not contain any references, especially when quoting DoE and the cost of the feel cell USD/KW. These are very technical things therefore they should be backed by references.

And the model proposed in this study exclusively on PEMFC and therefore the title should be PEMFC no the general fuel cell term.

Spelling mistake of degradation in the Figure. 5,6. Similarly in Figure. 4 it should be efficiency instead of “efficience”.

Please provide detailed explanations on how the catalyst recycling and recycling process contribute to the GHP.

Author Response

First, I would like to thank you for your thorough review of our paper (cleantechnol-3854961) and helpful comments to improve it.

 

Reviewer 3

Comments to the Authors

My comments are:

1. Authors pointed out mostly the cost analysis of the catalysts, however, the fuel cell cost comprises of other components such as membranes, bipolar plates and sometimes metallic bipolar plates. Please also take these factors into consideration.
2. Authors must support their claims with the sufficient literatures, entire introduction of the paper doesn’t not contain any references, especially when quoting DoE and the cost of the feel cell USD/KW. These are very technical things therefore they should be backed by references.
3. And the model proposed in this study exclusively on PEMFC and therefore the title should be PEMFC no the general fuel cell term.
4. Spelling mistake of degradation in the Figure. 5,6. Similarly in Figure. 4 it should be efficiency instead of “efficience”.
5. Please provide detailed explanations on how the catalyst recycling and recycling process contribute to the GHP.

 

Quality of English Language

(x) The English could be improved to more clearly express the research.

 

To Reviewer 3:

            Thank you for your review and valuable remarks.

 

1 Authors pointed out mostly the cost analysis of the catalysts, however, the fuel cell cost comprises of other components such as membranes, bipolar plates and sometimes metallic bipolar plates. Please also take these factors into consideration.

- We appreciate the reviewer’s insightful remark. In response, we have added a corresponding comment, where we acknowledge the contribution of other major components—such as membranes, bipolar plates, and metallic bipolar plates—to the overall fuel cell cost. This addition ensures that the cost analysis is more balanced and reflects the broader set of factors influencing the economic feasibility of fuel cell systems.

2. Authors must support their claims with the sufficient literatures, entire introduction of the paper doesn’t not contain any references, especially when quoting DoE and the cost of the feel cell USD/KW. These are very technical things therefore they should be backed by references.

- We thank the reviewer for this important observation. The Introduction section has been thoroughly revised to include appropriate citations supporting all technical statements, particularly those related to the DoE reports and the cost values of fuel cells in USD/kW. In total, 20 relevant references from recent literature and authoritative sources have been added, thereby strengthening the scientific foundation of the manuscript and ensuring that all claims are properly substantiated.

3. And the model proposed in this study exclusively on PEMFC and therefore the title should be PEMFC no the general fuel cell term.

- We thank the reviewer for this precise remark. We agree that the scope of the study is specifically centered on Proton Exchange Membrane Fuel Cells (PEMFCs). In response, we have revised the manuscript title accordingly to reflect this focus and avoid generalization.

Revised Title:

“Sustainability and Life Cycle Assessment of PEM Fuel Cells: From Materials to End-of-Life”

4. Spelling mistake of degradation in the Figure. 5,6. Similarly in Figure. 4 it should be efficiency instead of “efficience”.

- Thank you very much for the comment and clarification. I agree with you. I have made the appropriate corrections.

5. Please provide detailed explanations on how the catalyst recycling and recycling process contribute to the GHP.

- We thank the reviewer for this valuable suggestion. A new paragraph has been added in the Literature Review (Fuel Cell LCA) section, where the contribution of catalyst recycling and the recycling process to the overall Global Heating Potential (GHP) is explained in detail. This addition clarifies how recycling reduces primary resource extraction, lowers associated emissions, and thus significantly impacts the life cycle environmental assessment of PEM fuel cells.

Comments on the Quality of English Language

- The manuscript has been substantially reviewed and edited.

 

Thank you very much for your careful and thorough reading of our manuscript. Thank you very much for your remarks and comments. They were very helpful for us to highlight the main tasks and contributions of the manuscript, as well as to direct the readers' attention to the new and unique elements.

Reviewer 4 Report

Comments and Suggestions for Authors

1. The article presents some interesting results; however, the overall presentation is poor. It needs to be rewritten, and all figures should be revised to conform to standard article formatting.
2. Section 1 of the introduction, which provides a general overview of fuel cell technology and history, lacks even a single reference. It is essential to cite proper sources for the information presented.
3. Figures need to be revised for better clarity. For example figure 01, it is not clear where the data begins and ends.
4. The introduction lacks sufficient motivation for choosing PEFFC over SOFC as the focus of the study.
5. In lines 170–171, the author states that this model can be applied to SOFC. Considering the significantly different operational temperatures and conditions, the author should justify this claim in more details.
6. Wouldn’t it be more appropriate to present the I–V and I–P curves in a single plot (e.g., combining Figures 2 and 3) for better readability for readrs?
7. The manuscript requires careful revision for English language issues, as there are multiple spelling errors, such as those in the caption of Figure 6.
8. In Figures 5 and 6, the x-axis label should be changed from “Hour (h)” to “Time (h).”
9. The power degradation is modeled as linear, but in real laboratory experiments, material restructuring often leads to rapid initial performance loss before stabilization. How does the author intend to capture this effect in the modeling approach?
10. Figures must be carefully checked to avoid unit errors. For example, in Figure 9 the x-axis units for current are incorrect, if it represents current density, the correct unit is A/m². A similar issue is found in both the x- and y-axes of Figure 10.

Author Response

First, I would like to thank you for your thorough review of our paper (cleantechnol-3854961) and helpful comments to improve it.

 

Reviewer 4

Comments to the Authors

1. The article presents some interesting results; however, the overall presentation is poor. It needs to be rewritten, and all figures should be revised to conform to standard article formatting.
2. Section 1 of the introduction, which provides a general overview of fuel cell technology and history, lacks even a single reference. It is essential to cite proper sources for the information presented.
3. Figures need to be revised for better clarity. For example figure 01, it is not clear where the data begins and ends.
4. The introduction lacks sufficient motivation for choosing PEFFC over SOFC as the focus of the study.
5. In lines 170–171, the author states that this model can be applied to SOFC. Considering the significantly different operational temperatures and conditions, the author should justify this claim in more details.
6. Wouldn’t it be more appropriate to present the I–V and I–P curves in a single plot (e.g., combining Figures 2 and 3) for better readability for readrs?
7. The manuscript requires careful revision for English language issues, as there are multiple spelling errors, such as those in the caption of Figure 6.
8. In Figures 5 and 6, the x-axis label should be changed from “Hour (h)” to “Time (h).”
9. The power degradation is modeled as linear, but in real laboratory experiments, material restructuring often leads to rapid initial performance loss before stabilization. How does the author intend to capture this effect in the modeling approach?
10. Figures must be carefully checked to avoid unit errors. For example, in Figure 9 the x-axis units for current are incorrect, if it represents current density, the correct unit is A/m2. A similar issue is found in both the x- and y-axes of Figure 10.

 

 To Reviewer 4:

            Thank you for your review and valuable remarks.

 

1. The article presents some interesting results; however, the overall presentation is poor. It needs to be rewritten, and all figures should be revised to conform to standard article formatting.

- We sincerely thank the reviewer for this important feedback. The manuscript has undergone a substantial revision to improve its overall presentation, flow, and readability. All figures have been thoroughly reviewed, reformatted, and corrected to conform with standard article formatting. In addition, several sections of the manuscript have been rewritten and expanded to provide greater clarity, coherence, and completeness. These improvements ensure that both the text and the figures meet the expected standards of scientific publication.

2. Section 1 of the introduction, which provides a general overview of fuel cell technology and history, lacks even a single reference. It is essential to cite proper sources for the information presented.

- We appreciate the reviewer’s observation. In response, additional references have been incorporated into Section 1 of the Introduction to provide proper support for the general overview of fuel cell technology and history. These citations ensure that the background information is well-substantiated and aligned with established literature.

3. Figures need to be revised for better clarity. For example figure 01, it is not clear where the data begins and ends.

- We thank the reviewer for this helpful remark. Figure 1 has been replaced with a revised version that improves clarity, and a new, more precise title has been added to better reflect its content. In addition, a new explanatory paragraph has been included in the manuscript to clearly describe the figure and indicate where the data begins and ends. These changes ensure that the figure is now more informative and easier to interpret.

4. The introduction lacks sufficient motivation for choosing PEFFC over SOFC as the focus of the study.

- We thank the reviewer for this valuable comment. To address it, we have added a new paragraph in the Introduction providing a clear justification for focusing on PEMFC rather than SOFC. The added text highlights the advantages of PEMFC in terms of operating temperature, system compactness, start-up time, and suitability for integration into mobile and distributed applications, which align with the objectives of this study. This addition strengthens the rationale behind the chosen scope of the manuscript.

5. In lines 170–171, the author states that this model can be applied to SOFC. Considering the significantly different operational temperatures and conditions, the author should justify this claim in more details.

- We thank the reviewer for this important observation. The sentence in lines 170–171 has been revised with a more specific justification regarding the possible adaptation of the model to SOFC. The updated text clarifies that while the fundamental electrochemical equations remain applicable, modifications are required to account for the significantly higher operational temperatures, different electrolyte properties, and system configurations of SOFC. This revision provides a more accurate and nuanced explanation of the model’s applicability beyond PEMFC.

6. Wouldn’t it be more appropriate to present the I–V and I–P curves in a single plot (e.g., combining Figures 2 and 3) for better readability for readrs?

- We thank the reviewer for this valuable suggestion. Following the recommendation, Figures 2 and 3 have been combined into a single plot to present both the I–V and I–P curves together, thereby improving readability and comparison. The figure numbering has been rearranged throughout the manuscript to ensure consistency with this change.

7. The manuscript requires careful revision for English language issues, as there are multiple spelling errors, such as those in the caption of Figure 6.

- We thank the reviewer for this observation. All identified spelling errors, including those in the captions of Figures 3, 4, 5, and 6, have been corrected. In addition, the entire manuscript has undergone a comprehensive language review and editing process to eliminate typographical errors and improve clarity, readability, and consistency.

8. In Figures 5 and 6, the x-axis label should be changed from “Hour (h)” to “Time (h).

- We thank the reviewer for the suggestion. The x-axis label in Figures 5 and 6 has been corrected from “Hour (h)” to “Time (h),” ensuring accuracy and consistency in figure labeling.

9. The power degradation is modeled as linear, but in real laboratory experiments, material restructuring often leads to rapid initial performance loss before stabilization. How does the author intend to capture this effect in the modeling approach?

- We thank the reviewer for this valuable observation. A new paragraph has been added in the manuscript clarifying that the assumption of linear degradation is applied as a first-order approximation to facilitate simplified LCOE calculations. We acknowledge that real degradation profiles are typically nonlinear, with an initial rapid decline followed by stabilization. Future work will extend the model to include such nonlinear degradation dynamics, but for the scope of the present study, the linear approximation provides a practical and computationally efficient basis for techno-economic assessment.

10. The power degradation is modeled as linear, but in real laboratory experiments, material restructuring often leads to rapid initial performance loss before stabilization. How does the author intend to capture this effect in the modeling approach?

- We thank the reviewer for this important remark. In addition to the clarification already provided regarding the use of a linear approximation, all related figures have been reviewed and refined to ensure that the degradation trends are presented as clearly and accurately as possible within the chosen modeling framework.

 

Thank you very much for your careful and thorough reading of our manuscript. Thank you very much for your remarks and comments. They were very helpful for us to highlight the main tasks and contributions of the manuscript, as well as to direct the readers' attention to the new and unique elements.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed the comments in detail. The revised version is well-organized. One thing that needs to be corrected is that there are several duplicated figures, for example, in Figure.4. The authors may also want to recheck for any typo, like the title of Figure 4, fuel cell not fuell cell.

Author Response

First, I would like to thank you for your thorough review of our paper (cleantechnol-3854961) and helpful comments to improve it.

 

Reviewer 2

Comments to the Authors
The authors have addressed the comments in detail. The revised version is well-organized. One thing that needs to be corrected is that there are several duplicated figures, for example, in Figure.4. The authors may also want to recheck for any typo, like the title of Figure 4, fuel cell not fuell cell.

 

 

 

 

To Reviewer 2:

            Thank you very much for your review and valuable remarks.

 

1. The authors have addressed the comments in detail. The revised version is well-organized. One thing that needs to be corrected is that there are several duplicated figures, for example, in Figure.4. The authors may also want to recheck for any typo, like the title of Figure 4, fuel cell not fuell cell.

- We sincerely thank the reviewer for the careful reading and positive evaluation of our revised manuscript. We carefully reviewed all figures and corrected the duplications (e.g., Figure 4). Additionally, the typo in the caption of Figure 4 has been corrected (“fuel cell” instead of “fuell cell”), along with a full recheck for any other typographical errors throughout the text. We believe these corrections have improved the clarity and overall quality of the paper.

 

Thank you very much for your careful and thorough reading of our manuscript. Thank you very much for your remarks and comments. They were very helpful for us to highlight the main tasks and contributions of the manuscript, as well as to direct the readers' attention to the new and unique elements.

 

Reviewer 4 Report

Comments and Suggestions for Authors

• For example, lines 45–54 and 64–73 still lack any citations. This needs to be checked throughout the article. There is abundant literature on electrochemical energy conversion. The authors should include appropriate references. For guidance, see https://doi.org/10.1021/acsami.3c17766 and other key works.
• Figure 2 still has incorrect x-axis units. It is also unclear whether the x-axis represents current or current density. In addition, the figure shows one I–V and two I–P curves, which are confusing and not properly explained.
• Figure 3 still contains incorrect axis labels and an incomplete caption. The labels and units must be corrected, and the caption should clearly describe what is plotted.
• The manuscript continues to confuse current (A) with current density (A·m^-2). This issue was raised in the previous review but has not been addressed. The entire manuscript must be revised for consistent and correct use of current vs current density.

Despite the authors' previous round of review and revision, the manuscript still contains major deficiencies. Given that my earlier comments were not adequately addressed and the manuscript remains far from publication quality, I recommend rejection. The work may be reconsidered only as a new submission after substantial rewriting, correction of figures, and proper inclusion of recent and relevant literature.

 

Author Response

First, I would like to thank you for your thorough review of our paper (cleantechnol-3854961) and helpful comments to improve it.

 

Reviewer 4

Comments to the Authors:

 

For example, lines 45–54 and 64–73 still lack any citations. This needs to be checked throughout the article. There is abundant literature on electrochemical energy conversion. The authors should include appropriate references. For guidance, see https://doi.org/10.1021/acsami.3c17766 and other key works.

Figure 2 still has incorrect x-axis units. It is also unclear whether the x-axis represents current or current density. In addition, the figure shows one I–V and two I–P curves, which are confusing and not properly explained.

Figure 3 still contains incorrect axis labels and an incomplete caption. The labels and units must be corrected, and the caption should clearly describe what is plotted.

The manuscript continues to confuse current (A) with current density (A·m-2). This issue was raised in the previous review but has not been addressed. The entire manuscript must be revised for consistent and correct use of current vs current density.

Despite the authors' previous round of review and revision, the manuscript still contains major deficiencies. Given that my earlier comments were not adequately addressed and the manuscript remains far from publication quality, I recommend rejection. The work may be reconsidered only as a new submission after substantial rewriting, correction of figures, and proper inclusion of recent and relevant literature.

 

 

To Reviewer 4:

            Thank you for your review and valuable remarks.

 

1. 1. For example, lines 45–54 and 64–73 still lack any citations. This needs to be checked throughout the article. There is abundant literature on electrochemical energy conversion. The authors should include appropriate references. For guidance, see https://doi.org/10.1021/acsami.3c17766 and other key works.

- We thank the reviewer for this valuable comment and for kindly suggesting additional literature. In the revised version, we have added 4 new references in the sections indicated (lines 45–54 and 64–73). Furthermore, during the first revision we already included 20 additional references related to electrochemical energy conversion. We are grateful for the reviewer’s recommendation of further sources, which helped us to strengthen the manuscript.

2. Figure 2 still has incorrect x-axis units. It is also unclear whether the x-axis represents current or current density. In addition, the figure shows one I–V and two I–P curves, which are confusing and not properly explained.

- We thank the reviewer for pointing this out. The text has been revised for clarity and all typographical errors have been corrected. The dimensions have been unified between the methodology and the figures, and Figure 2 has been updated to clearly indicate that the x-axis represents current density. The explanation of the I–V and I–P curves has also been refined to avoid confusion.

3. Figure 3 still contains incorrect axis labels and an incomplete caption. The labels and units must be corrected, and the caption should clearly describe what is plotted.

- Thank you for the helpful remark. We have corrected the axis labels and units in Figure 3. We also harmonized all notation with the Materials and Methods section and added a brief note at the start of Results to explain the axis conventions used throughout the figures. We believe these changes remove any ambiguity and improve readability.

4. The manuscript continues to confuse current (A) with current density (A·m-2). This issue was raised in the previous review but has not been addressed. The entire manuscript must be revised for consistent and correct use of current vs current density.

- We appreciate the reviewer’s persistence in highlighting this important point. In the revised manuscript, we have clarified that all simulations are carried out with respect to current density. To avoid confusion, we explicitly state that the methodology section follows SI units (A·m⁻²), while the figures are presented in A·cm⁻² for better readability, with clear notation of the unit in each axis label. A final consistency check of the entire manuscript (figures, captions, and text) has also been performed to ensure correct and uniform use of current versus current density throughout.

5. Despite the authors' previous round of review and revision, the manuscript still contains major deficiencies. Given that my earlier comments were not adequately addressed and the manuscript remains far from publication quality, I recommend rejection. The work may be reconsidered only as a new submission after substantial rewriting, correction of figures, and proper inclusion of recent and relevant literature.

- We sincerely thank the reviewer for the detailed and constructive assessment of our manuscript. We acknowledge that in the previous round the final revised version was unfortunately not uploaded due to our oversight, and we apologize for this misunderstanding. In the present submission, the manuscript has been carefully and substantially revised: figures have been corrected, terminology has been unified, and additional recent and relevant literature has been included. We greatly appreciate the reviewer’s comments, which have helped us to significantly improve the quality of the paper, and we hope that the current version will now meet the publication standards.

 

Thank you very much for your careful and thorough reading of our manuscript. Thank you very much for your remarks and comments. They were very helpful for us to highlight the main tasks and contributions of the manuscript, as well as to direct the readers' attention to the new and unique elements.

 

 

Round 3

Reviewer 4 Report

Comments and Suggestions for Authors

The authors have clearly put effort into revising the manuscript, and I acknowledge the improvements made in figures, terminology, and the addition of references. The focus on PEMFCs is appropriate and aligned with the study’s objectives. However, I still have reservations about the overall quality and readiness of the paper for publication. In its current form, the manuscript requires further substantial revision before it can be considered.

1. The references in the introduction section do not match the interpretation of the background data. For example, reference 09 is about dry reforming of methane. After reviewing the cited article, it does not address electrolysis for hydrogen production, CO₂ reduction, or comparisons with batteries in rechargeability. Instead, it focuses on catalyst development for the fuel electrode. This indicates some references are misplaced or not directly relevant. The introduction must be revised with accurate and contextually appropriate citations.
2. The introduction lacks a clear flow and currently reads like a patchwork of different sources. It needs to be rewritten more coherently and structured to guide the reader logically from general background, to challenges, to the motivation and aims of the present study.
3. In line 75, the authors state that SOFCs are energy-intensive. However, high-temperature processes are thermodynamically favorable and often lead to higher efficiencies, as discussed in the following references:
• https://doi.org/10.3390/en17112603
• https://doi.org/10.1021/acsami.3c17766
  The authors should comment on this apparent contradiction. Additionally, in line 129, SOFCs are discussed again but in a different and somewhat inconsistent manner.
4. Starting from line 71, the authors present PGMs in a negative light (ethical mining, cost, etc.), which is a valid perspective. However, in line 133, the discussion seems to contradict this stance. The section needs rewriting for clarity and consistency to avoid conflicting interpretations.
5. Although the authors have adopted the correct term "current density” (A·cm^-2), several figure axes still incorrectly display the unit as “current [A.cm^-2].” This is confusing for readers and must be corrected to Current Density [A.cm^-2] throughout all figures and captions to ensure accuracy and consistency.

Author Response

Reviewer 4:

The authors have clearly put effort into revising the manuscript, and I acknowledge the improvements made in figures, terminology, and the addition of references. The focus on PEMFCs is appropriate and aligned with the study’s objectives. However, I still have reservations about the overall quality and readiness of the paper for publication. In its current form, the manuscript requires further substantial revision before it can be considered.

1. The references in the introduction section do not match the interpretation of the background data. For example, reference 09 is about dry reforming of methane. After reviewing the cited article, it does not address electrolysis for hydrogen production, CO2 reduction, or comparisons with batteries in rechargeability. Instead, it focuses on catalyst development for the fuel electrode. This indicates some references are misplaced or not directly relevant. The introduction must be revised with accurate and contextually appropriate citations.
2. The introduction lacks a clear flow and currently reads like a patchwork of different sources. It needs to be rewritten more coherently and structured to guide the reader logically from general background, to challenges, to the motivation and aims of the present study.
3. In line 75, the authors state that SOFCs are energy-intensive. However, high-temperature processes are thermodynamically favorable and often lead to higher efficiencies, as discussed in the following references:

        https://doi.org/10.3390/en17112603

        https://doi.org/10.1021/acsami.3c17766

        The authors should comment on this apparent contradiction. Additionally, in line 129, SOFCs are discussed again but in a different and somewhat inconsistent manner.

4. Starting from line 71, the authors present PGMs in a negative light (ethical mining, cost, etc.), which is a valid perspective. However, in line 133, the discussion seems to contradict this stance. The section needs rewriting for clarity and consistency to avoid conflicting interpretations.

5    Although the authors have adopted the correct term "current density” (A·cm-2), several figure axes still incorrectly display the unit as “current [A.cm-2].” This is confusing for readers and must be corrected to Current Density [A.cm-2] throughout all figures and captions to ensure accuracy and consistency.

 

 

 

To Reviewer 4:

            Thank you for your review and valuable remarks.

 

1. The references in the introduction section do not match the interpretation of the background data. For example, reference 09 is about dry reforming of methane. After reviewing the cited article, it does not address electrolysis for hydrogen production, CO2 reduction, or comparisons with batteries in rechargeability. Instead, it focuses on catalyst development for the fuel electrode. This indicates some references are misplaced or not directly relevant. The introduction must be revised with accurate and contextually appropriate citations.

- The entire Introduction and Literature Review sections have been thoroughly rewritten following a comprehensive re-evaluation of all cited references to ensure full consistency between the background discussion and the supporting sources. Incorrect or contextually misplaced citations — including those focused on catalytic processes unrelated to electrolysis, hydrogen production, or rechargeability comparisons between hydrogen and battery systems — have been removed.

Two new, contextually relevant references [8] and [9] have been added. These address electrochemical processes associated with CO₂ electroreduction and hydrogen generation within solid oxide and proton exchange membrane fuel cells. The revised Introduction now focuses exclusively on studies that are methodologically and thematically aligned with the scope of this paper, emphasizing:

• Life Cycle Assessment (LCA) and Life Cycle Sustainability Assessment (LCSA) frameworks applied to fuel cells;
• Techno-Economic Assessment (TEA) approaches integrating environmental and cost indicators;
• The influence of hydrogen origin (“grey”, “blue”, “green”) on environmental and energy efficiency metrics;
• Degradation mechanisms in PEMFC and SOFC systems;
• Circular economy principles, including platinum catalyst recycling and membrane recovery.

This restructuring ensures that every citation now supports a coherent scientific narrative and that the introductory section accurately reflects the state-of-the-art knowledge base. All newly included references are recent (2023–2025) and directly relevant to the manuscript’s focus on LCA–TEA integration and sustainability of fuel cell technologies.

These revisions address the reviewer’s concern by ensuring that all cited works are directly relevant and methodologically consistent with the paper’s objectives.

2. The introduction lacks a clear flow and currently reads like a patchwork of different sources. It needs to be rewritten more coherently and structured to guide the reader logically from general background, to challenges, to the motivation and aims of the present study.

- The entire Introduction and Literature Review sections have been completely rewritten to establish a coherent and logical flow. The revised version now guides the reader systematically — starting from the general background and current state of fuel cell technologies, through the main challenges related to life cycle assessment (LCA), techno-economic assessment (TEA), and material criticality, and finally leading to the specific motivation, objectives, and research questions of the present study.

Special attention has been paid to improving the narrative continuity and avoiding fragmentation between subsections. Transitional sentences were introduced to ensure smooth connections between topics such as system boundaries, hydrogen origin, degradation mechanisms, and circular economy strategies. The resulting structure provides a consistent progression from context → problem → motivation → aims → methodological framework.

These revisions fully address the reviewer’s concern by transforming the introduction from a compilation of separate references into a unified and logically structured overview that clearly motivates the scope and contribution of this study.

3. In line 75, the authors state that SOFCs are energy-intensive. However, high-temperature processes are thermodynamically favorable and often lead to higher efficiencies, as discussed in the following references:

        https://doi.org/10.3390/en17112603

        https://doi.org/10.1021/acsami.3c17766

        The authors should comment on this apparent contradiction. Additionally, in line 129, SOFCs are discussed again but in a different and somewhat inconsistent manner.

- The two suggested references have been added ([8] and [9]) and have replaced citations from earlier manuscript versions that were not fully appropriate for the discussed context. These works explicitly address the thermodynamic advantages of high-temperature operation in Solid Oxide Fuel Cells (SOFCs) and were incorporated to clarify the previously ambiguous statement regarding their energy intensity.

The text has been revised to explain that while SOFC systems require elevated operating temperatures (typically 700–1000 °C), these conditions are thermodynamically favorable and lead to higher overall efficiencies compared to low-temperature systems. The notion of “energy-intensive” now refers only to the material and manufacturing stages (e.g., ceramic processing, sealing, and sintering), rather than to operational inefficiency.

Furthermore, the discussion in line 129 was harmonized with this correction to ensure consistency throughout the manuscript. Both mentions of SOFCs now emphasize their dual nature — higher thermal efficiency during operation but greater embodied energy during production — providing a balanced and technically accurate interpretation.

These revisions address the reviewer’s concern by removing the contradiction, refining terminology, and aligning all SOFC-related statements with the cited thermodynamic evidence.

4. Starting from line 71, the authors present PGMs in a negative light (ethical mining, cost, etc.), which is a valid perspective. However, in line 133, the discussion seems to contradict this stance. The section needs rewriting for clarity and consistency to avoid conflicting interpretations.

- We sincerely thank the reviewer for the careful reading of the manuscript and for identifying this inconsistency. The Introduction and Literature Review sections have been rewritten to eliminate the conflicting interpretations regarding Platinum Group Metals (PGMs).

The revised text now maintains a consistent and balanced perspective: the environmental and ethical challenges related to PGM extraction and supply risks are acknowledged, while their indispensable role in achieving high electrochemical performance and durability in fuel cells is also emphasized. This dual framing reflects the current scientific consensus — recognizing PGMs as both critical and strategically sensitive materials whose sustainable management (through recycling, substitution, and circular design) is essential for long-term deployment of PEMFC and SOFC technologies.

These revisions address the reviewer’s concern by ensuring conceptual consistency and presenting a clear, unified narrative about the role of PGMs within the context of sustainability and performance.

5. Although the authors have adopted the correct term "current density” (A·cm-2), several figure axes still incorrectly display the unit as “current [A.cm-2].” This is confusing for readers and must be corrected to Current Density [A.cm-2] throughout all figures and captions to ensure accuracy and consistency.

- We thank the reviewer for this valuable recommendation. All figures and captions have been carefully revised to ensure that the correct unit and terminology — Current Density [A·cm⁻²] — are used consistently throughout the manuscript. The revised figures now accurately reflect that the analyzed parameter refers to current density, not total current, ensuring both technical precision and visual consistency across the manuscript.

These corrections fully address the reviewer’s comment and improve clarity and uniformity in all graphical elements.

 

Thank you very much for your careful and thorough reading of our manuscript. We sincerely appreciate your insightful remarks and constructive comments. They were very helpful in refining the structure and clarity of the paper, highlighting its main contributions, and drawing the readers’ attention to the novel and distinctive elements of our study.

 

Round 4

Reviewer 4 Report

Comments and Suggestions for Authors

accepted for publication in the present form