Metallic Materials for Hydrogen Storage—A Brief Overview

Round 1

Reviewer 1 Report

The review is helpful for an overall understanding of the basic theory, the methods for designing, and properties of light metal alloys and high-entropy alloys for hydrogen storage. However, revision is still necessary before the manuscript can be suggested for publication. The main comments are as the follows:

1. Importantly representative Figures in terms of the improved hydrogen absorption capacities of HEA in the previous reported works are suggested to summarize in the review for a better visible understanding.

2. Improvement of the organization structure of the section “Manufacturing processes (methods) for increasing absorption capacity” is necessary. The authors are suggested not to simply present a work from one research group in one paragraph, and then another. Summary of the main factors and the mechanisms for the increase of the absorption capacity is more expected for a review.

Author Response

Thank you for the review of the article and the factual comments that will contribute to its higher quality.

A language correction was made by a person with a C1 certificate.

Point 1: Importantly representative Figures in terms of the improved hydrogen absorption capacities of HEA in the previous reported works are suggested to summarize in the review for a better visible understanding.

Response 1: The appendix provides a summary of the individual hydrogenation properties of the discussed HEAs, (Table A1, Table A2, Table A3).

Point 2: Improvement of the organization structure of the section “Manufacturing processes (methods) for increasing absorption capacity” is necessary. The authors are suggested not to simply present a work from one research group in one paragraph, and then another. Summary of the main factors and the mechanisms for the increase of the absorption capacity is more expected for a review.

Response 2: Modifications have been made to Chapter 4 as per your recommendation.

Reviewer 2 Report

1. Introduction needs to be revised showing importance of metallic particles for Hydrogen storage.

2.Conclusion needs to show future perspective of the work.

3.As a review article, it needs to be more comprehensive, more recent references and findings need to add.

4. Graphical representation needed for overall presentation of the work.

5. Methods for Alloys composite section needs to be more comprehensive; it is too short.

6. Recent significant findings should be tabulated.

Author Response

Thank you for your careful review of the article and factual comments that will contribute to its higher quality. A language correction was made by a person with a C1 certificate.

Point 1: Introduction needs to be revised showing importance of metallic particles for Hydrogen storage.

Point 2: Conclusion needs to show future perspective of the work.

Response 1 and 2: The introductory chapter has been reworked and expanded, as has the conclusion.

Point 3: As a review article, it needs to be more comprehensive, more recent references and findings need to add.

Response 3: We elaborated the issue of hydrogen storage in the solid phase in more detail. We also expanded the literature and incorporated it into the text.

Point 4: Graphical representation needed for overall presentation of the work.

Response 4 and 6: Two graphic diagrams have been added to the text. An overview of the presented alloys and their hydrogenation properties are summarized in the tables in the appendix, (Table A1, Table A2, Table A3).

Point 5: Methods for Alloys composite section needs to be more comprehensive; it is too short.

Response 5: The chapter on design of alloy composition has been expanded.

6. Recent significant findings should be tabulated.

 

Reviewer 3 Report

The manuscript briefly reviews the developments in HEAs as hydrogen sorbents and the authors have presented it well. However, I'm unable to relate this work within the scope of the Coatings journal. Authors may look into this.

Further, there are minor typographical errors that need to be corrected.

1. Replace Mg2H with MgH2

2. Replace Paulling with Pauling (P. No.4)

It would be better if authors consider including studies on physisorption-based hydrogen adsorbents like MOFs and zeolites.

 

Author Response

Thank you for the review of the article and the factual comments that will contribute to its higher quality. A language correction was made by a person with a C1 certificate.

Point 1: Replace Mg2H with MgH2

Point 2: Replace Paulling with Pauling (P. No.4)

Response 1 and 2: Based on your comments, errors in the text have been corrected.

Point: It would be better if authors consider including studies on physisorption-based hydrogen adsorbents like MOFs and zeolites.

Response: In the introduction, other materials suitable for hydrogen storage were detailed, even on a non-metallic basis.

Reviewer 4 Report

The manuscript present enough content to be accepted in the present form. I do not see any points that need to be corrected or changed. Therefore, I believed that the journal audience will appreciate reading the manuscript as soon as it get published.

Author Response

Thank you very much for your time and review of the article.

Reviewer 5 Report

The current manuscript includes a brief review about the storage of hydrogen in metallic materials. The information have been organized in 5 different sections, including: general introduction, design of alloy composition, absorption properpies, manufacturing processes and conclusions. Depite the potential interest of the addressed topic in the work, I suspect that publication in the journal Coatings is not the best alternative, just readers of the journal focus their interests on preparation or testing layers incorporated onto a certain support (because that the name of the journal). Therefore, I suggest to move the publication to other journal of greater relevance for the discussed matter.

Moreover, some important points should be reconsidered before sending the new manuscript:

1. In general, the manuscript is too short to be a good review. Much more information should be included in a precise manner.

2. In the introduction section, I suggest to compare the use of metal alloys for hydrogen storage with other competitive alternatives in a better way. Authors only talk about gaseous, liquid and solid H2 storage in a general manner, but they don't discuss in dept other very relevant alternatives. In this context, it should be included here: LOHCs and porous solid materials (like zeolites or MOFs). Plese, consider the possible mobility of storage hydrogen during discussion of all the possibilities.

3. While describing potential metal alloys to favor the hydrogen storage, a table summarizing most relevant results would be very helpfuly for readers. No clear information for readers is prodived if interesting data are disseminated within the manuscript body text.

4. A similar approach should be followed when addressing the manufacturing of materials.

5. Please, revise the use of references and numbering throughout the entire manuscript.

6. Conclusions section should include a deep comment about current limitations, chalenges and future trends in this field.

Author Response

Thank you for your careful review of the article and factual comments that will contribute to its higher quality. A language correction was made by a person with a C1 certificate.

Point 1: In general, the manuscript is too short to be a good review. Much more information should be included in a precise manner.

Response 1: All chapters have been revised and expanded. A summary chapter was added.

Point 2: In the introduction section, I suggest to compare the use of metal alloys for hydrogen storage with other competitive alternatives in a better way. Authors only talk about gaseous, liquid and solid H2 storage in a general manner, but they don't discuss in dept other very relevant alternatives. In this context, it should be included here: LOHCs and porous solid materials (like zeolites or MOFs). Plese, consider the possible mobility of storage hydrogen during discussion of all the possibilities.

Response 2: In the introduction, among other things, the issue of porous solid materials (like zeolites or MOFs) was added.

Point 3: While describing potential metal alloys to favor the hydrogen storage, a table summarizing most relevant results would be very helpfuly for readers. No clear information for readers is prodived if interesting data are disseminated within the manuscript body text.

Response 3: The inclusion of individual hydrogenation properties presented by the HEA is given in the Appendix, (Table A1, Table A2, Table A3).

Point 4: A similar approach should be followed when addressing the manufacturing of materials.

Response 4: Chapter 4 was supplemented and better structured.

Point 5: Please, revise the use of references and numbering throughout the entire manuscript.

Response 5: Individual links are linked with references.

Point 6: Conclusions section should include a deep comment about current limitations, chalenges and future trends in this field.

Response 6: The conclusion was extended.

Round 2

Reviewer 5 Report

Authors submit again the manuscript after a first round of revision, including some improvements derived from reviewers comments. In fact, some issues have been clearly improved, although the work still maintain a critical drawback that avoid its acceptance for publication in this journal.  Basically, this fact is provoked by the absence of any mention about coating applications, being a general overview about metallic materials for hydrogen storage, without considering any application of these materials as coating, not being durectly used themselves.

In this context, the section of the journal in which the work has been includes states "Energy production and storage represent some of the leading issues facing contemporary society. The production of highly efficient materials for energy applications, such as photovoltaics, hydrogen production/storage, energy harvesters, thermoelectrics, and others, keep pushing the field of protective and functional coatings to new horizons. Motivated by the growing overlap between coating technology and energy fields, this Section of multifunctional Coatings for Energy Applications has been assembled. This Section aims to cover manuscripts combining rigorous coating technology development with novel applications in energy-related fields. We welcome contributions that provide clear improvements in designing, understanding, integrating, and developing energy production/storage coatings. Manuscripts dealing with organic or inorganic materials are also considered. Articles dealing with synthesis and characterization might be considered, as long as a relevant application in energy fields is clearly described."

Therefore, the current study should be rejected again definitely unless a complete restructuration to include explicitely the use of these materials in coatings. Thecniques for the deposition of these metals and the adherence between layers should be also considered in a worthy review.

Anyway, the current work could be accepted for publication in other journal not just focused on coatings. 

Author Response

Dear reviewer, Thank you for your positive and helpful feedback. Clearly, our topic is not specifically related to coatings. 
However, within our e-mail communication, we were directly addressed by the managing editor, Ms. Diana Zhao, who evaluated our article as suitable and invited us to submit it to "Coatings" journal. We gladly agreed to her offer. 
Our article is to be included in the special issue, which is also devoted to the storage of energy from hydrogen; so, it fits thematically.