Review Reports
- Péter Németh1,
- Botond Mecséri1,* and
- Barna Hanula2
Reviewer 1: Anonymous Reviewer 2: Salih Özer Reviewer 3: Anonymous
Round 1
Reviewer 1 Report
Comments and Suggestions for Authors1. The research gap is not well documented. The manuscript claims that "there is no literature comparing the seven hydrogen-based power systems" (page 3), but Table 1 only shows the Scopus search results and does not conduct a systematic literature review. It is necessary to supplement key research in the past five years and clearly state the incremental contribution of this study. The current statement is easily questioned for its lack of innovation.
2. Durability research is lacking. The original paper only compares macroscopic GWP (Table 1) but fails to consider the impact of membrane electrode degradation and catalyst poisoning on the entire lifecycle. When fuel cell efficiency decreases due to degradation, increased hydrogen consumption can lead to an underestimated GWP. The authors are encouraged to refer to 10.3390/en17123050 for further improvement.
3. Dynamic parameters are missing. The study assumes that the carbon intensity of electricity is static (e.g., electricity used for green hydrogen electrolysis is all from wind and solar power), but the actual decarbonization of the power grid is a dynamic process. Sensitivity analysis needs to be added to verify the robustness of the conclusions.
4. Key data are not traceable. The manuscript claims that FT vehicles have "50% lower" carbon emissions than hydrogen vehicles (abstract), but no data sources are provided: (1) FT synthesis efficiency assumptions; (2) biomass raw material carbon accounting methods. Original experimental data or authoritative databases must be cited.
5. The coupling mechanism between thermal and water management is lacking. The introduction mentions the need for high-purity hydrogen for PEMFCs (page 4), but ignores the issues of water flooding and membrane drying caused by thermal management imbalances, which can exacerbate performance degradation and affect efficiency assumptions in the LCA. Some recent work about water and thermal management can be added to further explain this issue, such as Energies 2023, 16(16), 6010, thermal management of liquid-cooled proton exchange membrane fuel cell: a review.
6. The conclusions are contradictory and unresolved. The abstract states that "H₂FCEV is optimal for the blue hydrogen scenario, and FT is optimal for the green hydrogen scenario", but the underlying reasons are not explained: (1) How does the CCS capture rate of blue hydrogen (90%?) and the carbon intensity of green electricity interact to influence the results; (2) Does the high energy density of FT fuel offset the synthesis loss? An attribution analysis model needs to be established.
7. Economic analysis is missing. As a policy-guiding study, the cost factor is not discussed: the LCOH (levelized cost of hydrogen) of green hydrogen electrolysis is currently 4-6€/kg, much higher than that of grey hydrogen (1.5€/kg), which will directly affect the priority of technology implementation. It is recommended to add a simplified LCC (life cycle cost) comparison chart.
8. Lifespan prediction models are inadequate. The finding in the conclusions, "Synthetic fuel vehicles are overtaking in the green hydrogen energy pathway," (page 1), lacks sensitivity analysis of the uncertainties in fuel cell lifetimes. Existing LCA models default to a 15-year lifetime, but actual operating lifespans fluctuate significantly. It is recommended to refine the analysis by referring to dynamic operating conditions, such as health state estimation and long-term durability prediction for vehicular PEM fuel cell stacks under dynamic operational conditions.
Author Response
Dear Reviewer,
We would like to sincerely thank you for your thorough evaluation of our manuscript and for the constructive recommendations you provided. We have carefully considered all of your comments, and they have been highly valuable in strengthening the quality and clarity of the paper. Below, we provide a detailed response to each of your points and describe the revisions made accordingly.
1."The research gap is not well documented. The manuscript claims that "there is no literature comparing the seven hydrogen-based power systems" (page 3), but Table 1 only shows the Scopus search results and does not conduct a systematic literature review. It is necessary to supplement key research in the past five years and clearly state the incremental contribution of this study. The current statement is easily questioned for its lack of innovation."
The research gap lies in the absence of a unified comparative perspective across alternative powertrains, a perspective that can not only provide more balanced environmental insights but also serve as a foundation for deeper technical evaluations, guiding future design, optimization, and policy development. We have added sources into the mentioned paragraph so the novelty of the study is more outlined. (See in the revised manuscript page 3)
2."Durability research is lacking. The original paper only compares macroscopic GWP (Table 1) but fails to consider the impact of membrane electrode degradation and catalyst poisoning on the entire lifecycle. When fuel cell efficiency decreases due to degradation, increased hydrogen consumption can lead to an underestimated GWP. The authors are encouraged to refer to 10.3390/en17123050 for further improvement."
Introducing a detailed durability and degradation assessment for fuel cell components, as suggested by Meng et al.'s focus on specific voltage degradation prediction techniques for PEMFCs, would necessitate a similar level of in-depth durability research for all other comparative technologies within the scope of our study. This would include, for example, the long-term degradation effects on diesel engines, other E-fuel combustion systems, or battery electric vehicle components. Such comprehensive degradation modeling would be crucial to maintain a consistent and fair basis for comparison across all evaluated powertrains, as their respective performance over time could also affect their overall life cycle emissions and consumption patterns.
3. "Dynamic parameters are missing. The study assumes that the carbon intensity of electricity is static (e.g., electricity used for green hydrogen electrolysis is all from wind and solar power), but the actual decarbonization of the power grid is a dynamic process. Sensitivity analysis needs to be added to verify the robustness of the conclusions."
We appreciate the Reviewer’s insightful remark regarding the dynamic nature of electricity decarbonization. Indeed, the carbon intensity of the grid will change over time, and this has implications for future life cycle assessments. However, the current study was deliberately designed as a comparative baseline framework rather than a predictive model. In order to maintain consistency and comparability across all pathways, a static carbon intensity was assumed, representing a fully renewable “green” case. This approach is aligned with previous LCA studies that establish methodological clarity before introducing temporal variability. Moreover, the principal findings of our paper, namely the relative ranking of vehicle technologies under different hydrogen production routes, are largely independent of the dynamic decarbonization trajectory. While absolute emission levels will undoubtedly decrease as the grid becomes cleaner, the relative differences between propulsion technologies remain robust, since all hydrogen derived fuels are equally affected by lower grid carbon intensity. We acknowledge that incorporating dynamic projections, for example 2030 or 2050 EU grid mixes, would be a valuable extension, but it falls outside the scope of this paper. Our intention is to first provide a unified comparative framework; we regard the proposed sensitivity analysis as an important future research direction, which can build upon the methodological foundation presented here.
4. "Key data are not traceable. The manuscript claims that FT vehicles have "50% lower" carbon emissions than hydrogen vehicles (abstract), but no data sources are provided: (1) FT synthesis efficiency assumptions; (2) biomass raw material carbon accounting methods. Original experimental data or authoritative databases must be cited."
The abstract is referring to the end results of the study it self H2FCEV and H2ICEV [51,47 and 59,89 gCO2/km] compared to the FT fuels' end results [26,6 and 32,22 gCO2/km]. There is no biomass in the study's examined FT fuel pathways. The energy efficiecny of FT synthesis in the case of eFuels is 57,47% "Energy efficiency ([E in fuel products]/[E in H2 and electricity inputs])" Weaved into the revised version of the manuscript as recommended. 5. "The coupling mechanism between thermal and water management is lacking. The introduction mentions the need for high-purity hydrogen for PEMFCs (page 4), but ignores the issues of water flooding and membrane drying caused by thermal management imbalances, which can exacerbate performance degradation and affect efficiency assumptions in the LCA. Some recent work about water and thermal management can be added to further explain this issue, such as Energies 2023, 16(16), 6010, thermal management of liquid-cooled proton exchange membrane fuel cell: a review." Thank you for highlighting the role of water and thermal management in PEMFCs. While the present study focuses on life-cycle GWP and does not model operational performance effects in detail, we have added a brief discussion in the revised manuscript referencing recent literature (e.g., Energies 2023, 16(16), 6010) to acknowledge how thermal imbalances and water management challenges, such as flooding or membrane drying, can influence efficiency and fuel cell durability. This provides context without expanding the scope beyond the comparative LCA focus.6. "The conclusions are contradictory and unresolved. The abstract states that "H₂FCEV is optimal for the blue hydrogen scenario, and FT is optimal for the green hydrogen scenario", but the underlying reasons are not explained: (1) How does the CCS capture rate of blue hydrogen (90%?) and the carbon intensity of green electricity interact to influence the results; (2) Does the high energy density of FT fuel offset the synthesis loss? An attribution analysis model needs to be established."
The reason those two alternatives are optimal is because of their whole life cycle's GWP indicators. They perform better than the other alternatives. Our sensitivity analysis shows an almost perfect inverse relationship (Pearson r ≈ –0.98) between CCS capture efficiency and total fuel cycle GHG emissions for H2FCEV and for FTDICEV aswell. At 90% capture, emissions remain significantly higher (~90 gCO₂e/km) than at 96–99% capture (~76–80 gCO₂e/km) for the H2FCEV and at 90% capture, emissions remain substantially higher (~134 gCO₂e/km) than at 96–99% capture (~104–111 gCO₂e/km) for the FTD ICEV. Therefore, while these results are insightful, a full sensitivity analysis is not included in the revised manuscript, as it would require examining every pathway and numerous additional parameters, shifting the focus away from the comparative GWP assessment.
7."Economic analysis is missing. As a policy-guiding study, the cost factor is not discussed: the LCOH (levelized cost of hydrogen) of green hydrogen electrolysis is currently 4-6€/kg, much higher than that of grey hydrogen (1.5€/kg), which will directly affect the priority of technology implementation. It is recommended to add a simplified LCC (life cycle cost) comparison chart."
Techno-economic analysis will be done with a different model in mind in a more specific research project, but in this case we have collected LCC data for conventional gasoline, diesel, and electricity for reference, 3 different hydrogen forms, RNG (e-methane), e-methanol, and synthetic diesel. 2030 and 2050 projections from different sources. 2050 projections for fossil roadfuels are too speculative to count in in this simplified case.
8. "Lifespan prediction models are inadequate. The finding in the conclusions, "Synthetic fuel vehicles are overtaking in the green hydrogen energy pathway," (page 1), lacks sensitivity analysis of the uncertainties in fuel cell lifetimes. Existing LCA models default to a 15-year lifetime, but actual operating lifespans fluctuate significantly. It is recommended to refine the analysis by referring to dynamic operating conditions, such as health state estimation and long-term durability prediction for vehicular PEM fuel cell stacks under dynamic operational conditions."
You are right to flag fuel cell lifetime uncertainty, but our statement does not concern fuel cell passenger cars. The synthetic fuel vehicles referenced are internal combustion vehicles operated on e fuels made with green hydrogen and captured carbon. As such, the conclusion is driven by fuel cycle emissions and energy inputs of efuel synthesis and use, rather than by assumptions about fuel cell stack durability. Moreover, the functional unit is per kilometer, and the relative ranking we report is dominated by upstream pathways and tank to wheel performance; fuel cell lifetime only affects the manufacturing and replacement burdens of fuel cell vehicles, which are not part of the e fuel ICE pathway considered here. We will clarify this scope explicitly in the conclusions and add a brief robustness note indicating that reasonable variations in fuel cell lifetime do not alter the stated result, while acknowledging that dynamic lifetime modeling is an important extension for studies focused on fuel cell vehicles.
We trust that the revisions and clarifications presented here address all of your recommendations and contribute to a clearer and more rigorous manuscript. We remain very grateful for your insightful feedback and for the opportunity to further improve our work. (Please see the provided revised manuscript, with the highlighted changes, thank you!)
Sincerely,
Botond Mecséri
Reviewer 2 Report
Comments and Suggestions for AuthorsResearchers state that they are conducting a Life Cycle Assessment (LCA) study that comparatively examines the life cycle impacts of energy obtained from hydrogen, seen as a next-generation energy source, through different production pathways (gray, blue, green) on various vehicle propulsion systems. For this purpose, the following issues need to be carefully addressed.
The summary of the article sufficiently reflects the scope and findings of the study. The summary should be strengthened by including the method used, the systems compared, and the main results.
In the introduction section, the existing literature should be comprehensively addressed. The aspects that differentiate this study from similar ones in the literature should be clearly defined, and if necessary, a literature review should be presented in a table format in the article. In this regard, the following articles can be examined: 10.1016/j.ijhydene.2024.10.394, 10.3390/en17112603.
The aim of the LCA study is emphasized as "determining the technology with the lowest GWP value." However, in addition to this goal, many objectives such as energy security, technological feasibility, and policy are addressed simultaneously in the introduction. This situation may create confusion for the reader. Therefore, this section should be reorganized.
Some types of vehicles, such as RNGV (Renewable Natural Gas Vehicles), are not clearly defined. Specifically, corrections should be made in the text by specifying the fuel production chain, biomass source, and system boundaries.
Why were conventional diesel/gasoline vehicles and BEVs selected as the comparison group? The scientific and political justification for this choice should be presented clearly.
The CO₂ emission data provided in the introduction (e.g., for the period 1990–2019) were evaluated based on ratios, but the comments made without absolute value analysis are misleading to the reader. This confusion needs to be resolved.
Color-coded hydrogen classifications are not sufficiently explained. Rarely used categories such as orange, purple, and red have been introduced technically but have not been related to their place in the literature.
Were the explanations regarding the non-mobility uses of hydrogen (e.g., industry, heating, fertilizer) presented in alignment with the main purpose of the article, or do they deviate from the focus?
Technical expressions regarding hydrogen usage (e.g., "high energy transfer") have not been quantitatively supported. The NOₓ emissions or efficiency values of H₂ICE engines should be provided, and this part of the article should be supported.
Have the references to significant projects like HYICE been assessed in context, or are they only mentioned at the name level?
The concept of HydS (Hydrogen Square) should be explained to the reader, and the content of the four-corner model should be clearly defined, allowing for meaningful graphical interpretation.
For each of the LCA steps (goal-scope, inventory analysis, impact assessment, interpretation), the approach and assumptions used should be clearly defined.
Is the functional unit of 200,000 km valid for all vehicle types? The validity of this assumption should be questioned in developing systems (e.g., FT ICEV, RNGV).
Has the direct applicability of the GREET model used been questioned in the context of the EU? Are there examples and sources that describe how this US-focused model has been parameterized?
Has the type of electrolysis used in hydrogen production (PEM, alkaline, etc.) and the energy sources (wind, solar, grid mix) been clearly defined?
In which source are the vehicle material components in Table 3 based? According to which manufacturer are the rare earth elements in BEVs and the carbon fiber additives in FCEVs modeled?
The fuel consumption values in Table 4 are given in simple units like L/100 km; what thermal values and formulas were used for these conversions? Has the background of these calculations been presented?
Is there a clear reference to the tables within the text? (e.g., "See Table 3") Additionally, are the table headings supported by the literature?
Has the study been limited solely to GWP (gCO₂e/km); should other environmental indicators (NOₓ, particulate matter, energy consumption, water footprint) be included in the assessment?
Have the negative CO₂ values of FT fuel systems (e.g., -357 and -431 gCO₂e/km) been explained along with the calculation method? Should the real-life applicability of these values be questioned?
Why have systems with gray hydrogen been included in the comparison? Has this been justified from the perspective of policy or transition technology despite their lower performance?
Only graphical information has been provided for RNGV systems; has the carbon balancing mechanism, energy chain, or environmental risks of this system been discussed?
Have the recycling effects remained merely at the level of commentary? In particular, have the recovered material ratios and impacts in FCEVs and BEVs been specified numerically?
Have the graphical data presented in Figures 2, 3, and 4 been tabulated? Is the presentation of numerical data necessary for the transparency of the evaluation from the reader's perspective?
Comments on the Quality of English LanguageThe technical terms, sentence repetitions, and sentence structures need to be reviewed.
Author Response
Dear Reviewer,
We would like to sincerely thank you for your thorough evaluation of our manuscript and for the constructive recommendations you provided. We have carefully considered all of your comments, and they have been highly valuable in strengthening the quality and clarity of the paper. Below, we provide a detailed response to each of your points and describe the revisions made accordingly.
1. "In the introduction section, the existing literature should be comprehensively addressed. The aspects that differentiate this study from similar ones in the literature should be clearly defined, and if necessary, a literature review should be presented in a table format in the article. In this regard, the following articles can be examined: 10.1016/j.ijhydene.2024.10.394, 10.3390/en17112603."
In the revised second half of the introduction, the literature review was extended with additional perspectives to better outline the novelty of the present work and to explicitly highlight the research gap that this study aims to address. The recommended articles proved highly valuable in structuring a more coherent and rigorous introduction.
2. "The aim of the LCA study is emphasized as "determining the technology with the lowest GWP value." However, in addition to this goal, many objectives such as energy security, technological feasibility, and policy are addressed simultaneously in the introduction. This situation may create confusion for the reader. Therefore, this section should be reorganized."
With the revision of the introduction, the evaluation based on GWP is given stronger emphasis, while the discussion of other factors is limited to brief mentions, as these aspects will be examined deeper in our later research projects.
3. "Some types of vehicles, such as RNGV (Renewable Natural Gas Vehicles), are not clearly defined. Specifically, corrections should be made in the text by specifying the fuel production chain, biomass source, and system boundaries."
Explanation for the RNG pathway has been added as recommended. The system boundaries are also added, the results on further inspection were changed using the 2024 revised version of the GREET 1 model. The new result data includes more realistic ratio of methane slip throughout the fuel production.
4."Why were conventional diesel/gasoline vehicles and BEVs selected as the comparison group? The scientific and political justification for this choice should be presented clearly."
Comparison groups justification has been clarified in the manuscript with added references, thank you for the recommendation! (See page 9)
5."The CO₂ emission data provided in the introduction (e.g., for the period 1990–2019) were evaluated based on ratios, but the comments made without absolute value analysis are misleading to the reader. This confusion needs to be resolved."
We fully agree that presenting both proportional and absolute data is necessary to avoid misinterpretation. In the original manuscript we already referred to absolute global values (20 Gt in 1990 and 34 Gt in 2019), but we realize that this point may not have been sufficiently emphasized. To address this, we have revised the Introduction and added a clarifying sentence explicitly stating the absolute growth of transport-related emissions (from approx. 4 Gt in 1990 to more than 7 Gt in 2019). This ensures that readers clearly see that, despite the relative share of the transport sector remaining nearly constant, its absolute emissions increased substantially in parallel with total global CO₂ emissions.
6."Color-coded hydrogen classifications are not sufficiently explained. Rarely used categories such as orange, purple, and red have been introduced technically but have not been related to their place in the literature."
To address the Reviewer’s concern, we have added a clarifying sentence after Table 2, explicitly noting that rarely used color categories (orange, purple, red) are presented only for completeness and are not part of our comparative LCA scenarios.
7."Were the explanations regarding the non-mobility uses of hydrogen (e.g., industry, heating, fertilizer) presented in alignment with the main purpose of the article, or do they deviate from the focus?"
9."Have the references to significant projects like HYICE been assessed in context, or are they only mentioned at the name level?" The reference to projects such as HYICE was included to illustrate the technological foundation and historical relevance of hydrogen internal combustion engine research in Europe. The purpose was not to provide a detailed project-level assessment, but rather to show that our LCA framework builds upon pathways that have already been technically explored and validated in practice. In this sense, the HYICE project demonstrates that hydrogen ICE technology has benefited from substantial R&D support and is therefore a credible option to include in the comparative analysis. While we agree that a more extensive review of specific projects would be valuable, it would exceed the scope of this article, which is focused on comparative life cycle impacts. For this reason, the projects are briefly referenced by name and role, rather than assessed in depth.
10. "The concept of HydS (Hydrogen Square) should be explained to the reader, and the content of the four-corner model should be clearly defined, allowing for meaningful graphical interpretation."
The Hydrogen Square (HydS) was included to illustrate the systemic challenges of hydrogen technology beyond its production, namely storage, safety, and end-use integration. To improve clarity, we have revised the manuscript by adding a short explanation of the four-corner model, defining each dimension of the square and its role in assessing hydrogen pathways. This ensures that the figure can be meaningfully interpreted and directly linked to the overall framework of our study.
11."For each of the LCA steps (goal-scope, inventory analysis, impact assessment, interpretation), the approach and assumptions used should be clearly defined."
Added our approach clearly to the LCA part in the revision as recommended. (See page 10)
12. "Is the functional unit of 200,000 km valid for all vehicle types? The validity of this assumption should be questioned in developing systems (e.g., FT ICEV, RNGV)."
The choice of a 200,000 km lifetime as the functional unit follows established LCA practice for passenger cars, ensuring comparability across different vehicle types. We fully agree that in the case of developing systems such as FT ICEVs and RNGVs, actual lifetime values may diverge due to technological immaturity, durability uncertainties, or limited large-scale deployment data. However, the purpose of our study was to provide a baseline comparative framework, and for consistency across scenarios it was essential to use a uniform functional unit. We would also emphasize that using 200,000 km for all vehicle types does not bias the relative comparison: any deviation in actual lifetimes would affect all drivetrain options proportionally when normalized to the functional unit.
13. "Has the direct applicability of the GREET model used been questioned in the context of the EU? Are there examples and sources that describe how this US-focused model has been parameterized?"
Most input parameters in GREET can be customized to reflect EU upstream emissions. Only the petroleum-based fuel pathways are more strongly structured around U.S. conditions, yet these too can be adjusted to align with European upstream data. Wong et al. uses GREET1 in an MPDI article for non-US contexts as mentioned in the introduction.
14."Has the type of electrolysis used in hydrogen production (PEM, alkaline, etc.) and the energy sources (wind, solar, grid mix) been clearly defined?" "The FT diesel ICEV has the best results overall and the least gross emissions throughout its lifetime, whose hydrogen feedstock was produced with green electric PEM water electrolysis" added wind power as used energy inside the model calculations. 15. "In which source are the vehicle material components in Table 3 based? According to which manufacturer are the rare earth elements in BEVs and the carbon fiber additives in FCEVs modeled?" The vehicle material components in Table 3 are based on the GREET2 model database, which provides default assumptions for vehicle structures and material breakdown. The rare earth elements in BEVs and the carbon fiber additives in FCEVs are likewise modeled according to GREET’s generic vehicle design data rather than a specific manufacturer, meaning they represent average or typical industry values rather than brand-specific configurations. Most BEVs utilize rare-earth permanent magnets in modern EV motors (https://thundersaidenergy.com/downloads/electric-vehicles-motors-and-magnets/), and most FCEVs rely on high-pressure, composite hydrogen storage Type IV vessels, where carbon fiber composites (carbon fiber overwrap plus a liner) form the bulk of structural material. (https://www.mdpi.com/2411-9660/7/4/97)
16."The fuel consumption values in Table 4 are given in simple units like L/100 km; what thermal values and formulas were used for these conversions? Has the background of these calculations been presented?" LHV was used harmonically in all the fuels' calculations, these can also be found in the GREET1 model. Added the mentioned backgroup information to the manuscript. (Please see page 11 of the revised manuscript) 17. "Is there a clear reference to the tables within the text? (e.g., "See Table 3") Additionally, are the table headings supported by the literature?" We have revised the text to provide greater emphasis on this point. The current section headings were intentionally structured to align directly with the input parameters of the GREET2 model, which allows for clear parametrization. The LCIA calculations for the vehicle cycle are subsequently derived from these material composition ratios.
18."Has the study been limited solely to GWP (gCO₂e/km); should other environmental indicators (NOₓ, particulate matter, energy consumption, water footprint) be included in the assessment?"
In future research it is planned to compare NOx, PM, and CEDnr, but upon detailed inspection, the GREET model's data in NOx and PM are slightly oversimplified in some phases (vehicle operation) so we will be taking other models to make the assessment. For a broad initial comparison, the GWP metric is deemed sufficient enough, as emphasized in the revision.
19. "Have the negative CO₂ values of FT fuel systems (e.g., -357 and -431 gCO₂e/km) been explained along with the calculation method? Should the real-life applicability of these values be questioned?" Yes, the negative CO₂ values arise because producing FT e-fuels requires CO₂ to be captured from power plants or other industrial sources, so the sequestration is credited within the LCA. However, their real-life applicability should be viewed critically, as the feasibility depends on large-scale, reliable CO₂ capture and integration with hydrogen production. 20. "Why have systems with gray hydrogen been included in the comparison? Has this been justified from the perspective of policy or transition technology despite their lower performance?" Grey hydrogen systems are included as a critical benchmark representing the current industrial baseline. Their inclusion is justified from a transition perspective, as they establish the reference point from which the environmental performance of green hydrogen and other synthetic fuel pathways must be evaluated. This comparison is essential for informing pragmatic policy that incentivizes the shift towards truly sustainable alternatives.
21."Only graphical information has been provided for RNGV systems; has the carbon balancing mechanism, energy chain, or environmental risks of this system been discussed?" The updated model now incorporates RNG methane slippage and a more realistic carbon loop, addressing gaps in the 2023 version. However, upon further review, these parameters were ultimately adjusted to align with the study's core focus on GWP (methane is taken into calculation of the CO2 equivalent indicator). 22. "Have the recycling effects remained merely at the level of commentary? In particular, have the recovered material ratios and impacts in FCEVs and BEVs been specified numerically?" The contribution of recycling remains marginal compared to phases like fuel production and vehicle use. The recovered material ratios and impacts for ICEVs, BEVs, and FCEVs are not our assumptions but are sourced directly from the European Commission's LCA report, which quantifies end-of-life effects and provides the numerical basis for our analysis.
23."Have the graphical data presented in Figures 2, 3, and 4 been tabulated? Is the presentation of numerical data necessary for the transparency of the evaluation from the reader's perspective?"
New revised figures have been added, and also added a table of numeric data to the appendix.
We trust that the revisions and clarifications presented here address all of your recommendations and contribute to a clearer and more rigorous manuscript. We remain very grateful for your insightful feedback and for the opportunity to further improve our work.
Sincerely,
Botond Mecséri
Reviewer 3 Report
Comments and Suggestions for AuthorsRespected Authors,
Your paper is written clearly, linguistically correct and based on a recognized methodology developed by UChicago Argonne, LLC. I did not find any errors that required linguistic or methodological correction.
However, the study relies entirely on an existing LCA model and does not provide sufficient original scientific input. It lacks innovative, own methodological elements, unique data sets or new comparative perspectives, which limits its importance for the development of scientific knowledge.
The introduction of these elements would require a fundamental redesign of the work and in fact, a rewriting of the paper. For this reason, despite its good presentation and formal correctness, I cannot recommend it for publication in a scientific journal such as Hydrogen. I have forwarded my comments to the Editor, who may have a different perspective on this work. I will respect any of his decisions.
Sincerely,
Reviewer.
Author Response
Dear Reviewer,
We respectfully thank the Reviewer for the time and effort invested in evaluating our manuscript. We acknowledge the concern raised about the originality of the study. However, we would like to emphasize that while the GREET model is indeed a well-established and widely used LCA framework, the scientific contribution of our work does not lie in modifying the model itself, but in applying it in a novel comparative context.
Specifically: - To the best of our knowledge, this is the first study to provide a unified comparative life cycle assessment of seven distinct hydrogen-based propulsion technologies (H₂FCEV, H₂ICEV, MeOH FFV, EtOH FFV, FT diesel ICEV, FT gasoline ICEV, RNGV), benchmarked against conventional ICEVs and BEVs. Previous studies typically address only one or two alternatives in isolation. - By synthesizing these pathways within a consistent cradle-to-grave framework, our work identifies relative rankings and trade-offs that were not visible in prior single-technology LCAs. This comparative perspective offers actionable insights for policymakers, technology developers, and the scientific community. - The inclusion of FT-based and RNGV pathways, which are rarely examined in passenger car LCAs, provides an original angle and extends the scope of hydrogen mobility research beyond the commonly studied FCEV vs. BEV comparison. In addition, we have carefully considered the constructive feedback from the other two Reviewers. All of their comments have been addressed and incorporated into the revised manuscript, including clarifications on emission data presentation, hydrogen color classification, the HydS model explanation, and the discussion of functional unit assumptions. We believe these improvements have significantly strengthened the manuscript. For these reasons, we respectfully submit that our study does provide new comparative perspectives and scientific value. While it does not introduce a new LCA methodology, it fills a recognized gap in the literature by bringing multiple hydrogen pathways into a single, consistent evaluation. We therefore hope that, upon reconsideration, the Reviewer will agree that the work offers a meaningful contribution to the field and is suitable for publication in Hydrogen.Sincerely,
Botond Mecséri and Péter Németh
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsI consider that the revised version of the manuscript can be accepted for publication, since the authors have made efforts to improve the manuscript by considering the reviewer's comments. They have addressed the issues pointed out and made adequate modifications.
Reviewer 2 Report
Comments and Suggestions for AuthorsI appreciate the authors for making the requested updates. The article is now prepared for publication.
Reviewer 3 Report
Comments and Suggestions for AuthorsRespected Authors,
I would like to thank the Authors for their detailed response to my previous comments. I accept clarification that the novelty of the work does not lie in the development of a new LCA methodology, but in conducting an integrated comparison of seven different hydrogen technologies within one coherent model. According to my knowledge, this type of analysis has not been presented in the literature so far and constitutes a valuable, synthesizing contribution.
I share the opinion that the results presented in the work may be of practical importance for decision makers, technology designers, because they enable transparent identification of trade-offs and relative rankings of technologies that were not visible in the existing, fragmentary analyses. I maintain the opinion that the scientific value of the work is limited, but the application achievements allow to conclude that the reviewed work is consistent with the profile of the Hydrogen journal.
I also appreciate the fact that the manuscript has already been supplemented and corrected in accordance with the suggestions of the other reviewers. The current version is clear, consistent and complete. I have no further comments from my side.
I recommend publishing the article in the Hydrogen journal.
Sincerely,
Reviewer