Green Hydrogen for Energy Transition: A Critical Perspective

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In section 1.1, hydrogen concepts are not clearly handled as an element and elementary molecule of hydrogen.

The similarity of the chemical process with the electrochemical process is described not adequately. In the last ones, two separate electrode reactions exist.

Section 2. It would be more appropriate not to put references in the section title and better place them just when they are mentioned within the paragraph.

Include a table in which, in addition to CO2 emissions per kg of hydrogen, the use of water by technology has to be described, that is, water for directly obtaining H2 and the water that is used for process (cooling water, etc. .), the required energy for the production of hydrogen per process and the cost of hydrogen per process.

Place the references corresponding to these data in the Figure Caption for Figure 1.

In section 3. Make a depth analysis of green hydrogen production methods, comparing their efficiencies, energy consumption, water, etc.

 

In section 3.1. Compare the electrolysis processes, Faradic Efficiencies, Costs per Kg of hydrogen, and purities obtained.

 

In section 3.2. Present a table with relevant data from the latest research on the development of membranes, such as materials, conductivity, durability, operating temperature, etc. Costs

 

In section 3.3. Indicates what characteristics must be achieved in the new materials to make them better for sun radiation (solar spectrum) use.

In section 4.3.9. Mention the reduction of CO2 emissions quantitatively or in percentages by incorporating hydrogen into the aforementioned projects.

 

In section 4.3.10. This is an interesting possibility of water recovery in fuel cells. How much water could be recovered by technology?

 

One of the questions of water used to generate hydrogen and use it in heat processes is the increase in the water footprint of products. Compare the water footprint of gasoline, aluminum, cement, steel, with the use of green hydrogen or without the use of it.

 

In section 4.4.3 Improve the description of the MOF. There are several crystalline materials with high surface area that allows physisorption and chemisorption of hydrogen. Notice the difference.

 

In section 4.4.5. Improve the description of electrochemical compression devices

In section 4.5. Indicate quantitative data of the projects to understand their dimensions. Concentrate this information on a table. Hydrogen production, technology for this, mixing percentages with natural gas, etc.

Indicate the H2 production capabilities of these projects and the production technology used (electrolysis type), energy consumption and water.

Indicate blending percentages in projects that use it

In section 6.1.1 Improve the description of the operation of a fuel cell.

In section 7. Indicate the use of water as a challenge because it competes with other human activities like agriculture or human use, and the need to incorporate to hydrogen technology the use of seawater.

 

In section 7.4. Address new water sources (seawater, treatment, etc.) and their economic and technical implications.

 Globally, review the manuscript so that it is not very general and avoid repeating concepts explained briefly, such as electrochemical compression.

Author Response

Reviewer 1

Comment and suggestions to the authors

Globally, review the manuscript so that it is not very general and avoid repeating concepts explained briefly, such as electrochemical compression.

Response

We appreciate Reviewer's valuable feedback, which provides essential insights into improving the manuscript. Below, we address each of the concerns raised and propose appropriate actions for revisions: 

1. Section 1.1: Clarification on Hydrogen as an Element and Molecule

Objection: In section 1.1, hydrogen concepts are not clearly handled as an element and elementary molecule of hydrogen.

The similarity of the chemical process with the electrochemical process is described not adequately. In the last ones, two separate electrode reactions exist.

Response

We have expanded the discussion to clearly differentiate hydrogen as an atomic and molecular species, highlighting its properties and significance in chemical and electrochemical contexts. Further, we have clarified the key differences in processes such as electrode reactions in electrochemical cells, detailing the mechanisms involved. See new text at lines 79-98.

2. Section 2: References and Data Table for Technologies

Objection: It would be more appropriate not to put references in the section title and better place them just when they are mentioned within the paragraph.

Include a table in which, in addition to CO2 emissions per kg of hydrogen, the use of water by technology has to be described, that is, water for directly obtaining H2 and the water that is used for process (cooling water, etc...), the required energy for the production of hydrogen per process and the cost of hydrogen per process. Place the references corresponding to these data in the Figure Caption for Figure 1.

Response

We acknowledge the reviewer for this observation. References have been relocated to the appropriate paragraphs and in the figure caption of fig.1 as well. By complying with the reviewer’s suggestion, a comparative table 1 has been added summarizing CO₂ emissions, direct and process water usage, energy consumption, and costs for each hydrogen production technology, with references cited in the table caption. See table 1 and new text at lines 191-200.

3. Section 3: Deep Analysis of Green Hydrogen Production Methods

Objection: Make a depth analysis of green hydrogen production methods, comparing their efficiencies, energy consumption, water, etc.

Response

The section has been extended by reporting efficiencies, energy demands, and challenges associated with various green hydrogen production methods. See new text at lines 208-219.

Objection: In section 3.1. Compare the electrolysis processes, Faradic Efficiencies, Costs per Kg of hydrogen, and purities obtained.

Response

The section has been revised to include detailed comparisons of production efficiencies, water and energy usage, and costs per kg of hydrogen. For electrolysis, we have analyzed technologies (PEM, AWE, SOE) based on Faradaic efficiency, operational costs, and output purity. Quantitative data have been included in a second summary table. See new text at lines 248-257.

4. Section 3.2: Membrane Development Details

Objection: Present a table with relevant data from the latest research on the development of membranes, such as materials, conductivity, durability, operating temperature, etc. Costs

Response

We appreciate and comply with the reviewer’s observation. We have included a detailed new table 3 alongside relevant references in the table caption, comparing membranes, focusing on material advancements, ionic conductivity, operational conditions, and cost efficiency. See new text at lines 283-291.

5. Section 3.3: Characteristics for Solar Spectrum Utilization

Objection: Indicates what characteristics must be achieved in the new materials to make them better for sun radiation (solar spectrum) use.

Response

Main characteristics required for membranes in solar spectrum utilization have been summarized and illustrated at lines 317-337 together with appropriate references.

6. Section 4.3.9 & 4.3.10: Quantitative CO₂ Reduction and Water Recovery

Objection: Mention the reduction of CO2 emissions quantitatively or in percentages by incorporating hydrogen into the aforementioned projects.

Response

Paragraph 4.3 has been completely reassembled. In its revised version, it is now divided into four sub-paragraphs, the first of which contains the bullet "Transport and mobility" along with new texts at lines 702-707 and 711-714 and showcasing data on CO₂ reductions achieved in the cited projects.

Objection: This is an interesting possibility of water recovery in fuel cells. How much water could be recovered by technology?

Response

At lines 803-816 we illustrate a quantitative estimation of water recovered through the production of green hydrogen via electrolysis on the basis of a hypothetical case study.

Objection: One of the questions of water used to generate hydrogen and use it in heat processes is the increase in the water footprint of products. Compare the water footprint of gasoline, aluminium, cement, steel, with the use of green hydrogen or without the use of it.

Response

In the revised paragraph 4.3, a new sub-paragraph 4.3.4 titled “Key factors influencing water footprint” has been added including a comparative analysis of the water footprint illustrated for several industrial products with and without green hydrogen. This analysis has been also summarized in a new table 4. See new text at lines 844-859.

7. Section 4.4.3 & 4.4.5: Improve Descriptions of MOFs and Electrochemical Compression

Objection: In section 4.4.3 Improve the description of the MOF. There are several crystalline materials with high surface area that allows physisorption and chemisorption of hydrogen. Notice the difference.

Response

We appreciate the reviewer for this comment. In the new version of the paper, the discussion on MOFs and electrochemical compression falls under the sub-paragraph “4.1.2. Electrochemical-based storage methods”. Here, we clearly differentiate physisorption and chemisorption mechanisms in MOFs, specifically highlighting the factors affecting the former mechanism. See new text at lines 489-508.

Objection: In section 4.4.5. Improve the description of electrochemical compression devices.

Response

According to the reviewer’s suggestion, the electrochemical compression devices have been explained in detail, focusing on their working principles and efficiency metrics. See new text at lines 521-538.

8. Section 4.5: Project Dimensions and Data

Objection: Indicate quantitative data of the projects to understand their dimensions. Concentrate this information on a table. Hydrogen production, technology for this, mixing percentages with natural gas, etc. Indicate the H2 production capabilities of these projects and the production technology used (electrolysis type), energy consumption and water. Indicate blending percentages in projects that use it.

Response

As the projects mentioned in the paper are organised in the form of multi-national projects, most of the required quantitative data is not directly available or accessible from the respective official websites. However, the comparative assessment of listed projects in terms of hydrogen production capacity, technology used, energy consumption and water, is not within the aims of this review article.

9. Section 6.1.1: Fuel Cell Operation

Objection: Improve the description of the operation of a fuel cell.

Response

This section has been expanded to describe fuel cell process, including electrode reactions, proton exchange processes, and the role of catalysts in generating electricity. See new text at lines 1116-1123.

10. Section 7: Water Use Challenges

Objection: Indicate the use of water as a challenge because it competes with other human activities like agriculture or human use, and the need to incorporate to hydrogen technology the use of seawater.

Response

In sub-section 7.1 we have addressed water competition in hydrogen production, highlighting seawater utilization technologies and their feasibility, including desalination and treatment costs. See new text at lines 1171-1179.

11. Section 7.4: New Water Sources

Objection: Address new water sources (seawater, treatment, etc.) and their economic and technical implications.

Response

We have discussed advances in seawater electrolysis, the technical challenges, and economic implications of incorporating alternative water sources into hydrogen production. See new text at lines 1244-1261.

12. General Suggestions

Objection: Avoid generalizations and repetition; focus on precise, detailed descriptions.

Response

The manuscript has been reviewed comprehensively to eliminate generalizations and repetitions. Repetitive discussions on electrochemical compression and other topics have been consolidated for clarity.

Reviewer 2 Report

Comments and Suggestions for Authors

The green hydrogen as one of the most attracted renewable candidates is of interesting to academic and industries workers. This work gives a perspective review on the green hydrogen, including bibliometric analysis and sketch points on the integrating with industry. The review work is suitable for publish in energies. Generally, this review is comprehensive enough but the description for each part is too concise to offer valuable information for readers. Some issues should be corrected before considered further published in this journal.

 1.        Summary figures should be provided than solely text to help the reader to well understand the layout of GH2 related topics, for examples, the cost analysis.

2.        The description of current methods for GH2 production is too generally without providing summary, for example, ratio of each technique.

3.        The ‘challenges’ for each method at section 3 is not proper as more overview on the technology itself should be given, such as the work principle, pros and cons, advanced development etc. Moreover, there is section 7 which is focus on the challenges.

4.        Please rearrange the layout of section 4, as some parts are overlapped, catharized into ‘Storage and integration’ should be good enough.

5.        Fig2 should be modified, i.e. the arrange of ‘HtC/F’ and ‘battery storage’ etc.

Author Response

Reviewer 2

Comment and suggestions to the authors

The green hydrogen as one of the most attracted renewable candidates is of interesting to academic and industries workers. This work gives a perspective review on the green hydrogen, including bibliometric analysis and sketch points on the integrating with industry. The review work is suitable for publish in energies. Generally, this review is comprehensive enough but the description for each part is too concise to offer valuable information for readers. Some issues should be corrected before considered further published in this journal.

Response

We sincerely appreciate Reviewer's thoughtful feedback, which provides an opportunity to refine and enhance our manuscript. All changes have been made to enhance the manuscript's clarity and comprehensiveness. We hope the revisions satisfactorily address all concerns. Below, we address each comment in detail and outline the corresponding revisions made.

1. Summary figures

Objection: Summary figures should be provided than solely text to help the reader to well understand the layout of GH2 related topics, for examples, the cost analysis.

Response

We agree that including summary figures can enhance clarity. To address this, we have incorporated a comprehensive diagram as new Figure 2, summarizing the percentages of both the efficiency and adoption ratio of various hydrogen production processes whereas the associated costs are listed in a new Table 1 as described in the successive point 2. Additional visual aids, such as numerical indicators illustrating technology readiness levels (TRL), have also been added to complement the chart. See new text at lines 381-407.

2. Description of Current Methods for GH2 Production

Objection: The description of current methods for GH2 production is too generally without providing summary, for example, ratio of each technique.

Response: This section has been expanded to include a detailed summary table 1 that presents greenhouse gas emissions, direct and process water usage, energy requirements and costs of hydrogen production methods, hopefully enabling readers to quickly grasp key points. The adoption ratio of each method is illustrated in the new figure 2 (see point 1 above). Supporting bibliometric analysis has been also included to create the table. See new text at lines 191-200.

3. Challenges for Each Method in Section 3

Objection: The ‘challenges’ for each method at section 3 is not proper as more overview on the technology itself should be given, such as the work principle, pros and cons, advanced development etc. Moreover, there is section 7 which is focus on the challenges.

Response

We have revised section 3 to include a summary overview focusing on the Faradaic efficiency, purity of hydrogen produced, costs, advantages and limitations of each electrolysis technology. In particular, a new explanatory table 2 has been added, which provides a comparative overview of the electrolysis processes for hydrogen production (see new text at lines 248-257). Moreover, paragraph 3.2 “Advanced membrane for electrolysis” has been integrated with a new table 3, which summarizes the main features provided by membranes for electrolysis technology such as materials, conductivity, durability, operating temperature and costs (see new text at lines 283-291). Section 3.3 “Photolysis” has been also updated to include the key characteristics needed for membranes to utilize solar radiation effectively (see new text at lines 317-337). These include broad-spectrum absorption, efficient charge separation and transport, high photocatalytic efficiency, selective permeability for gas separation, and cost-effectiveness with scalability. Finally, challenges related to specific methods have been relocated to Section 7 to avoid redundancy. Hopefully, section 3 now provides a structured, technology-focused narrative.

4. Rearrangement of Section 4

Objection: Please rearrange the layout of section 4, as some parts are overlapped, catharized into ‘Storage and integration’ should be good enough.

Response

Section 4 has been reorganized and renamed "Green hydrogen storage methods and grid integration" (line 412). The content overlap has been corrected to improve readability and consistency. Therefore, paragraph 4.4 has been merged with 4.1, together with the insertion of new bibliographical references. Besides, the resulting text has been subdivided into the sub-paragraphs: “4.1.1. Large scale storage” (lines 452-458), and “4.1.2. Electrochemical-based storage methods” (lines 459-483). Subsections now address specific aspects, such as storage technologies (e.g., underground caverns, adsorption in solid-state materials, gas compression) and integration with renewable energy systems. Finally, a new flow diagram (figure 3) has been added at the end of the new version of paragraph 4.1 to summarise the key points of hydrogen storage technologies. See new text at lines 538-551.

5. Modification of figure 2

Objection: Fig2 should be modified, i.e. the arrange of ‘HtC/F’ and ‘battery storage’ etc.

Response

Figure 2, which in the revised manuscript is identified as Figure 4, has been completely redesigned to reflect a more logical arrangement of components, such as Hydrogen-to-Chemicals/Fuels (HtC/F) and Battery Storage. Labels have been revised and caption has been shortened for clarity. The updated figure should provide now a better visual representation of energy flow within a sector-coupled smart grid.

Reviewer 3 Report

Comments and Suggestions for Authors

This article provides an overview of the recent pivotal role of green hydrogen (GH2) in energy conversion. It covers the classification, production processes, integration with the power grid, reuse methods post-integration, existing challenges, and improvement measures related to GH2. This work facilitates an understanding of the significant role GH2 is currently playing in the energy and industrial sectors. However, several issues persist within this article:

Figures:

1. This review contains only two figures, which is insufficient for a comprehensive overview.
2. The figure captions should provide concise key information rather than being excessively long.
3. The second figure exhibits several problems: the layout is chaotic; some words are outside the flowchart boxes; in the brown box, why does H2→chemical proceed through H2tF rather than H2tC; and why can't "hydro" in "power generation sources" be directly used for PtC?

Structure:

1. Sections 4.1 "Green hydrogen storage" and 4.4 "Electrochemical-based materials and methods for hydrogen storage" have overlapping content and should be merged.
2. Subsection 3.2.1 can be integrated into Section 3.2 without needing to be separately listed.
3. Section 4.3 can essentially be categorized into two types: using H2 as an energy source or as a raw material. This section's presentation could be simplified.
4. Section 1.2 "Energy per unit volume (consideration of challenges)" does not effectively demonstrate H2's high energy density and should be revised or removed.
5. Specific values should be provided for the "Bonding Energy" and "Combustion Energy".

Formations:

1. In lines 192 and 231, change the period "." after "challenge" to a colon ":".
2. The formats in lines 228 and 233 are inconsistent.
3. The subtitle classification format in Section 3.4 differs from those preceding it.
4. The abbreviation "PtX" first appears in line 321 without its full name being given.
5. The subtitle formats in lines 322 and 437 are inconsistent.
6. There is an issue with the paragraph format in line 719.
7. The first two paragraphs in Section 7.1 have a different layout from the subsequent text.
8. In line 865, the abbreviation for "Power-to-Hydrogen-to-Power" is "P2H2P," which is inconsistent with the previous similar abbreviation style "PtH2P".
9. In line 916, "HtP" should be changed to "H2tP" for clarity.
10. Section 8's layout differs from the preceding sections.
11. In line 1173, change the colon ":" to a period "." for consistency.
12. Lines 1178, 1182, and 1190 are inconsistent with the preceding text.
13. Many references lack volume and page number information.

Despite covering a broad range of topics, the article is somewhat subjective in content and lacks data support. For instance, Section 3, "Current methods of production of green hydrogen," introduces various processes but fails to provide specific efficiency numbers. Combined with the above issues, this review may not be suitable for publication in Energies.

 

Comments on the Quality of English Language

Very poor.

Author Response

Reviewer 3

Comment and suggestions to the authors

This article provides an overview of the recent pivotal role of green hydrogen (GH2) in energy conversion. It covers the classification, production processes, integration with the power grid, reuse methods post-integration, existing challenges, and improvement measures related to GH2. This work facilitates an understanding of the significant role GH2 is currently playing in the energy and industrial sectors. However, several issues persist within this article:

Response

We sincerely appreciate the referee’s thorough review of our manuscript and the constructive feedback he has provided. We appreciate his feedback and believe these revisions have significantly enhanced the quality and scientific rigor of our manuscript. We acknowledge the reviewer for is time and valuable insights. Below, we address each of his concerns in detail and outline the revisions we have made to improve the quality, clarity, and accuracy of our work.

Figures:

 1. Number of figures

Objection: This review contains only two figures, which is insufficient for a comprehensive overview.

Response

We acknowledge the need for additional visual elements and, accordingly, have included more diagrams and tables to provide a comprehensive overview of the key concepts discussed in the review.

2. Figure caption

Objection: The figure captions should provide concise key information rather than being excessively long.

Response

The figure captions have been revised to be more concise and informative while maintaining clarity.

3. Issues in Figure 2

Objection: The second figure exhibits several problems: the layout is chaotic; some words are outside the flowchart boxes; in the brown box, why does H2→chemical proceed through H2tF rather than H2tC; and why can't "hydro" in "power generation sources" be directly used for PtC?

Response

Figure 2, which in the revised manuscript is identified as Figure 4 (pag. 24), has been completely redesigned to reflect a more logical arrangement of components. In particular, the previous flow structure, which now assumes a circular symmetry, has been modified to ensure that "H₂-to-Chemicals (H₂tC)" is directly connected to “Power-to-Hydrogen (PtH2) rather than routed through "H₂-to-Fuel (H₂tF)". In addition, in the new version of the diagram, all primary energy sources appear globally linked to each of the various forms of energy utilization (PtX). Labels have been revised and caption has been shortened for clarity. The updated figure should provide now a better visual representation of energy flow within a sector-coupled smart grid.

Structure:

 4. Merging Sections 4.1 and 4.4:

Objection: Sections 4.1 "Green hydrogen storage" and 4.4 "Electrochemical-based materials and methods for hydrogen storage" have overlapping content and should be merged.

Response

We have combined these sections into a unified discussion on hydrogen storage technologies, eliminating redundancy and providing a clearer, more structured overview. The new paragraph 4.1 (line 438) has also been supplemented by a new flow diagram (figure 3) to summarise the key points of hydrogen storage technologies.

5. Subsection 3.2.1 Integration:

Objection: Subsection 3.2.1 can be integrated into Section 3.2 without needing to be separately listed.

Response

The content of subsection 3.2.1 (Flexible Electrolysis Operations) has been integrated directly into Section 3.2, removing the need for a separate listing. Moreover, a new explanatory table 2 has been added (lines 248-257), which provides a comparative overview of the electrolysis processes for hydrogen production.

6. Simplification of Section 4.3:

Objection: Section 4.3 can essentially be categorized into two types: using H2 as an energy source or as a raw material. This section's presentation could be simplified.

Response

We are very grateful to the reviewer for is constructive comment and accordingly, the section has been restructured into two categories: 4.3.1 “H₂ as an energy carrier” at line 646 (e.g., power generation, fuel for transportation, and grid balancing); 4.3.2 “H₂ as a feedstock” at line 716 (e.g., ammonia synthesis, methanol production, and refining processes). Moreover, a new subsection 4.3.4 “Key factors influencing water footprint” (line 843) has been added to compare the water footprint for several production processes in presence and absence of green hydrogen (see table 4).

7. Section 1.2 Revision/Removal:

Objection: "Energy per unit volume (consideration of challenges)" does not effectively demonstrate H2's high energy density and should be revised or removed.

Response

This section has been rewritten to more effectively demonstrate hydrogen’s energy density compared to other fuels. See text at lines 102-106 and 112-116.

8. Addition of specific values

Objection: Specific values should be provided for the "Bonding Energy" and "Combustion Energy".

Response

Bonding energy: A precise value has been included for the H₂ bond dissociation energy (436 kJ/mol). Combustion energy: The enthalpy of combustion of H₂ (−286 kJ/mol or 120 MJ/kg) has been explicitly stated for clarity. See lines 87 and 92.

Formations & Formatting Issues

We have carefully revised the manuscript to correct all formatting inconsistencies:

9. Punctuation Adjustments:

Objection: In lines 192 and 231, change the period "." after "challenge" to a colon ":".

Response

The substitution is not necessary because the text has been modified.

10. Inconsistent Formatting (Lines 228, 233, 322, 437, 719, 1173, 1178, 1182, 1190, and Section 8):

Response

These sections have been reviewed, and formatting has been made consistent throughout.

11. Standardization of Abbreviations:

Objection: The abbreviation "PtX" first appears in line 321 without its full name being given.

Response

"PtX" has been defined upon its first appearance in the manuscript.

12. In line 865, the abbreviation for "Power-to-Hydrogen-to-Power" is "P2H2P," which is inconsistent with the previous similar abbreviation style "PtH2P".

Response

"P2H2P" has been corrected for consistency with the notation used in other sections.

13. Reference Formatting:

Response

Missing volume and page numbers have been added where available. References have been cross-checked for accuracy.

14. Data support

Objection: Despite covering a broad range of topics, the article is somewhat subjective in content and lacks data support. For instance, Section 3, "Current methods of production of green hydrogen," introduces various processes but fails to provide specific efficiency numbers.

Response

We acknowledge the reviewer’s concern regarding the lack of quantitative data in some sections. To address this, in Section 3 specific efficiency values for electrolysis methods (e.g., PEM, AWE, SOE) have been incorporated and cost and energy consumption are provided in the comparative tables 2-3.

15. English Language Improvements

Response

We acknowledge the concern regarding the quality of English expression. The entire manuscript has undergone professional proofreading to improve clarity, readability, and grammatical correctness, rewording of complex or ambiguous sentences for better comprehension and consistency checks for terminology and technical expressions.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The requested corrections were made It is suggested acceptance for publication.

Reviewer 2 Report

Comments and Suggestions for Authors

The revised manuscript is recommended to be published

Reviewer 3 Report

Comments and Suggestions for Authors

This review is well written, and should be accepted for publication soon.