Water Electrolysis Technologies and Their Modeling Approaches: A Comprehensive Review

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript attempts to summarize the development and progress of water electrolyzers and to propose how numerical simulations can help to further the progress of this technology. For the former, the authors attempted to summarize the optimizations made to the water electrolyzers, including electrodes, supports, electrolyte and membrane. For the latter, the authors discussed how numerical simulations can assist in understanding the water electrolyzer system, with a particular focus on Finite Element Model (FEM) and its application in PEM electrolyzer systems. While review articles have been published on water electrolyzers, the main strength of this is highlighting the role of numerical simulations. The fact that the majority of the reference is dated before 2017 makes the relevance of this manuscript questionable since this does not seem to reflect the progress of water electrolysis technologies.

The following are the major weaknesses of this manuscript:

1. Too few recent publications (especially 2019 – 2024) were cited.
2. The length of the manuscript is excessive.
3. Particularly in Section 3, significant proportion of the works cited were merely mentioned without being adequately discussed. (For example, Lines 206 – 214 have mentioned a few modifications on the cathode of alkaline water electrolyzers (AWEs), but their relationship to performance improvement has not been discussed.) Although many of the subsections have a summarizing paragraph at the end, the readers were not able to see the insights of the authors on the topic. Readers expecting inspirations on strategies to improve their own technologies may not be able to gain from reading this review manuscript. The authors should consider providing mind map(s) to aid the readers.
4. There is no discussion or mention on Anion Exchange Membrane Water Electrolyzer (AEMWE) despite it being discussed in recent publications. The authors may want to include this in the discussion to increase the relevance of this manuscript.
5. The caption of figures (i.e., Figures 7 – 14) in Section 3 are lacking citations. The plots might have been made by the authors themselves, but the data source should be included in the caption so that readers will understand that the data comes from other publications.
6. Numerical simulations other than Finite Element Model (FEM) for PEM electrolysis are not adequately discussed. Given the title of the manuscript and how Section 3 aims to be inclusive, Section 5 should also include other numerical simulations other than FEM so that the readers can know the difference and to know which method should best be used for each water electrolyzer technology.

As such, it is recommended that this manuscript should not be accepted in its present form. It can be reconsidered if the manuscript has its scope and purpose redefined (e.g., how much of the current progress should be discussed, and whether it should discuss all numerical simulation methods or it should be limited to just FEM, for which it should be reflected in the title) and significant number of recent publications is included in the manuscript.

The following issues should be addressed as well:

7. Line 52: “clean energy production” should have been “storage for renewable-energy-derived electricity” as water electrolysis is not “energy production”.
8. Figure 1: Do the colored boxes in the concentric circles represent anything? It is also recommended that the prevalence of these methods be included in the figure if the numbers are available. The word “pyrolysis” may need more elaboration because it is too general when placed under “Other feedstocks”.
9. Table 2: Increasing time may increase or decrease the current density depending on the condition of the cell. Usually, with increasing time, the current density will decrease due to dissolution of catalysts at the electrode, membrane degradation, etc. It should be clarified if an increase in electrode surface area will increase current density since current density is defined by current per unit area of the electrode surface. The factor “concentration of ions” should also be clarified as different ions affect different electrodes differently. Also, catalyst loading in mg/cm² should also be considered as a factor.
10. Line 113: Relevance to the research objectives, study design, language, and quality of the study are evaluated for the second stage screening process. The authors should clarify how “relevance to the research objectives” and “quality of the study” are evaluated, and what type of study design and what language would be deemed relevant for the purpose of this evaluation.
11. Figures 3 and 4: Numbers or percentages should be included in the pie charts, especially the major components.
12. Figure 5b: The authors should clarify what is meant by high, medium and low temperatures.
13. Lines 217 – 218: The authors should clarify if the modified porous nickel coatings are for cathode or anode, and how an improvement in anodic efficiency would reduce the overpotential of both hydrogen evolution reaction and oxygen evolution reaction.
14. Lines 220 – 222: The authors should clarify how Reference 27 has contributed to performance improvement.
15. Lines 223 – 224: The authors should clarify what the plasma treatment was done on.
16. Figures 7 and 8: The authors should clarify how efficiency should be calculated (and to mention if it is faradaic efficiency, energy efficiency, etc.) and what are the experimental conditions under which these data are obtained (e.g., temperature, concentration of alkali, pH, current density). Any mention of efficiency in this manuscript should be accompanied by a mention of its type (or how it is derived).
17. Lines 300 – 302: Reference 52 has been mentioned for its replacement of the electrolyte with aqueous methanol to “lower voltage as compared to water electrolysis” and hence showing that “lower operating voltage and higher energy efficiency can be attained for water electrolysis”. The logic here is confusing as the stand of the authors on whether this system is still water electrolysis is unclear. Depending on the anode, the replacement of the electrolyte with aqueous methanol may cause the replacement of oxygen evolution reaction (OER) with methanol oxidation reaction (MOR), which, depending on the selectivity of the anode, may also emit CO₂. This will result in reduction in cell voltage as MOR potential is lower than that of OER. Hence, this is not exactly the optimization of water electrolysis, but a significant modification of the process such that it should be considered as a class of ‘water electrolyzers’ on its own. The authors should rethink how this reference is presented to prevent misleading the readers to think that it is merely an ‘additive’ effect.
18. Lines 315 – 317: The trends of current density and potential differences versus molarity in Figure 9e lack explanation, particularly on what the trends are and why.
19. Lines 327 – 329: The authors should explain how the proposed membranes are “more advantageous” than the commercial Zifron 500 utp membranes.
20. Lines 329 – 332: The mention of Ni electrodeposition on carbon paper does not fit in the paragraph that seems to be discussing about membranes. Although this is done in membrane-electrode assembly, this is clearly not a membrane improvement.
21. Lines 348 – 351: The authors should clarify on what the “model” is.
22. Lines 406 – 408: In an appropriate part of the manuscript, the authors should discuss the current benchmark of fidelity of numerical simulations like FEA and CFD.
23. Lines 445 – 446: The authors should mention the optimal Pt and Ir loadings derived from the study by Grigoriev et al.
24. Lines 449 – 453: The authors mentioned the experimental variation in Reference 71 but did not summarize the results obtained in the study.
25. Lines 455 – 457: The unit of degradation rate should have been “µV/h” and not “uV/h¹”. The authors should also clarify which anode is “180 uV/h¹” corresponding to.
26. Lines 488 – 489: “Mass activity at 1.6V/Amg” should have been “mass activity at 1.6 V”.
27. Lines 498 – 500: The first sentence in the paragraph mentioned the impact of membrane, but the discussion on Reference 80 about porous transport layer does not seem to be relevant to membranes.
28. Figure 11 is not well ordered and the text in the figure is not visible. Figure 11e is too small.
29. Lines 510 – 513: The authors should clarify if Reference 82 is indeed about Proton Exchange Membrane Water Electrolyzer (PEMWE).
30. Lines 530 – 531: The authors should clarify how Reference 90 is related to the discussion about varying pressure.
31. Lines 561 – 564: The authors should clarify what are the “perfluorosulfonic-acid-based ionomers” and how different are they from “conventional Nafion membranes” as Nafion is a perfluorosulfonic acid ionomer.
32. Figure 12: The authors should clarify the experimental conditions (i.e., control/constant variables) under which these data are obtained (e.g., anode, cathode, temperature, concentration of alkali, pH, current density, cell voltage, type of stability test).
33. Line 598: The authors should clarify the benefits of “H₂O+CO₂ co-electrolysis” especially when both water electrolysis and CO₂ electrolysis have two different cathodic reactions (i.e., hydrogen evolution reaction (HER) and CO₂ reduction), possibly competing for active sites. This is seen as a possible diversion from the main objective of water electrolysis, which is production of green hydrogen.
34. Lines 656 – 658: The authors should clarify why SFN-YSZ is more appropriate than Ni/YSZ for HER and the purpose for carbon deposition on Ni/YSZ.
35. Figure 13 and Figure 14 have not been meaningfully discussed in the text. The caption for Figure 14a is missing.
36. Lines 1070 – 1071: The authors should elaborate on what are the weaknesses or improvement strategies to be considered by the readers regarding “the H₂ and power integration strategies”.
37. Lines 1071 – 1072: The authors should elaborate on the “Cu-Cl cycle of H₂”
38. Line 1076: The authors should clarify what is meant by “A pretreated electrolysis of coal”, especially on what pretreatment is being done.
39. Lines 1144 – 1145: The authors should clarify their assumptions on the water electrolysis system to explain the limitations of the 3D cubic meshing of 32 × 32 × 32 elements.
40. Some elements in Equations 7, 9 and 10 have not been defined in the text. The authors should also explain the difference in the two “gas volume fraction” quantities ϕ and 𝜖.
41. Line 1177: The authors should clarify if cell overpotential has been neglected in this setting of boundary conditions.
42. Table 7: The authors should clarify the source of the values used for the parameters listed.
43. Lines 1193, 1215, 1232 and 1305: The authors should clarify the source of “experimental results”.
44. Line 1211: The phrase “where non-uniform current density distributions arise” should be visually clarified in Figure 15.
45. Figure 16: The red and blue planes in (b) need to be labelled. Experimental data should be included in the figure to demonstrate the alignment between the simulated data and the experimental data.
46. Line 1258: The authors should clarify “the electrode edges and flow channels” visually in Figure 18.
47. Table 8: The authors should clarify what the assumed system consists of (e.g., anode, cathode, membrane, electrolyte, temperature, active area).
48. Lines 1342 – 1344 and Figure 21: The authors may want to discuss possible sources of deviations. This is important because although the absolute deviation values seem to be small, there seems to be a trend of these values against the x-axis in the plots in Figure 21 (e.g., in Figure 21b, the deviation increases with increasing current density).

Comments on the Quality of English Language

1. Many abbreviations have been used without being mentioned in full form. For example, “YSZ” in Line 510, “EE” and “CE” in Line 881, “CC” in Line 887, “COD” in Line 917, “FEM” in Line 1099, and “RWGS” in Line 1361.
2. The phrasing for some of the sentences is confusing and needs to be revised, such as “the sea salt impregnated vaporized at 800 °C” with two verbs next to each other in Line 596 and “hydrogen degradation rates” in Line 1363 when it seems likely that it should have been “degradation of hydrogen production rate”.
3. Some typographical errors need to be corrected, for example “0.3-0.4 % kh” (instead of “0.3 – 0.4%/kh” in Line 637) and “Co₂ electrolysis mode” (Line 641).
4. It is suggested that the authors spend some time and resource to proofread the manuscript prior to future submissions as long manuscripts like this have higher tendency for language errors that reduce their readability.

Author Response

Please refer to the attached document for detailed responses to the reviewers' comments.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The review provides a comprehensive overview of water electrolysis technologies, including AWE, PEMWE, SOEC, and MEC. However, there is a lack of in-depth summary and induction on the mechanisms of various water electrolysis technologies as well as their advantages and disadvantages. We recommend acceptance of the paper pending minor revisions. The manuscript exhibits the following deficiencies:

 

1) The captions of the figures in the article are too simple and do not indicate the sources of relevant figures and data. It is recommended to add necessary annotations and explanations so that readers can better understand the content of the charts and trace the relevant content to ensure the accuracy of the data.

 

2) The image quality of Figure 9 and Figure 11 is not high enough. They are somewhat blurry and need to be improved in terms of clarity.

 

3) The article discusses the application of numerical modeling and Finite Element Analysis (FEA) in water electrolysis, but it lacks detailed descriptions of specific models and algorithms. It is suggested to provide more in-depth explanations of model assumptions, boundary conditions, and solution methods to help readers better understand the simulation process. In addition, no references were cited in this part of the content, and it seems more like the author's own research content from the perspective of the content.

 

4) The review lacks a comparative analysis between simulation results about FEA and experimental data. It is recommended to include comparisons between experimental data and simulation results to validate the accuracy and reliability of the models, and to discuss any discrepancies and their potential causes.

 

5) The conclusion section briefly mentions future research directions, but the discussion is relatively superficial. It is recommended to provide a more detailed exploration of potential future research directions, particularly addressing current technological bottlenecks and challenges, and to propose more forward-looking research suggestions.

 

6）There are some mistakes in the article. For example, "4.1" should be "5.1".

Author Response

Please refer to the attached document for detailed responses to the reviewers' comments.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The current revised manuscript has addressed some of the concerns highlighted in the previous round. However, some of the issues still need resolving, including:

1. Line 63: The word 'electrolysis' in should be replaced by 'water electrolysis' for clarity.
2. Table 2: 'Activation' should be mentioned in Row 2. The limit of increased electrolysis with applied voltage can also be due to mass transport and diffusion limitations. Effects of ion concentrations also depends on the membrane.
3. Especially in Section 3, paragraphing needs to be improved. For example, one paragraph should be dedicated to discussion of cathodes, and the next on the discussion of anodes, so that it can be more readable for the readers. Mentions of References 23 and 27 (for example) about catalysts at Lines 229 - 231 and Lines  238 - 240 seem to be out of place in that paragraph. Other similar issues should be rectified.
4. Figure 7: The range of experimental conditions (temperatures, current densities, KOH concentrations) is too wide for the data to be meaningfully compared in the figure. It is suggested that more data from similar experimental conditions be used for comparison instead.
5. Lines 306 - 334: References 43, 44 and 47 are cited for electrode improvement without mentioning whether anode or cathode has been improved. This should be clarified.
6. Lines 377 - 380: The benefit of water electrolysis at elevated temperature (beyond water boiling point) should be briefly explained.
7. Figures 7 and 8: The source of the data should be cited. 
8. Lines 423 - 454: The discussion/description of Figure 9 should be discussed in the previous paragraph, and not separately, as the purpose of figures in a review paper is to explain what is written in the text and not the other way round. (Similar issues for other figures such as Figures 12, 14 and 15 should be rectified.). There is a typographical error "0.5 NaCl" (Line 435) to be rectified. "SGP-RED" in Line 437 should be explained. The 'negative slope portion' in Line 440 seems to be the polarization of SGP-RED system, which has nothing to do with the electrolyzer. The authors should also confirm if the 'APE Water Electrolysis' system in Reference 62 is indeed Alkaline Water Electrolyzer or Anion Exchange Membrane Water Electrolyzer.
9. Since Figure 10 is about cathode optimization, discussions in Lines 483 - 492 should be placed right before the discussion of other components such as membranes and electrolytes.
10. Line 641 has a missing full stop.
11. The caption of Figure 12 has to be revised. Figure 12c is definitely not a plot of mass activity versus anode loading. The mention of units in the caption or in the text right after the parameter (e.g., "mass activity at 1.6V/Amg^-1" and "anode loading/mg cm^-2") is confusing and unnecessary. Figure 12e is overloading, and it is suggested that the most relevant of the 3 plots remain, while others should be removed. Figure 12 organization should be improved.
12. Line 861: A typographical error - "Avery". The whole manuscript should be checked for typographical errors, especially for extra parenthesis after the square brackets belonging to the citation numbers.
13. Line 1159: COD should be named in full at the first instance of usage and not in subsequent uses.
14. Line 1163: "methane yield (0.14 LCH₄/gCOD)". "CH₄" and "COD" should be in subscript.
15. Line 1445: The x in "CoCrOx" should be in subscript.
16. Lines 1475 - 1527: The authors can consider making these paragraphs more concise and more coherent. These paragraphs should describe major modelling methods, their strengths and limitations, and what the cited authors achieved with them. The authors should consider discussing Figure 23 here instead. The discussion of methods other than FEM is too limited to reflect the inclusivity in the revised title.
17. The authors should explain why FEM is the sole focus for the major part of Section 5.
18. Lines 1581 - 1593: These descriptions should be right after their respective equations, and not at the end of a sub-section.
19. Figure 16: Is the annotated steep potential gradient along the y-axis or along the x-axis? This should be clarified in the figure.
20. Lines 1823 - 1831: The authors explained the deviation at higher current densities in Figure 22b, but did not explain the deviation at the lower current densities in Figure 22c and Figure 22d. The authors should consider citing authoritative references to support their explanations too, if possible.
21. Details in Figure 23 should be backed by references, and hence it is more appropriate to be discussed before Sub-section 5.1. The "Comparative Analysis" branch should be removed from the figure and explained in the text, together with the strengths and limitations of each method.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

The revised manuscript has addressed many of the concerns highlighted in the previous round. The recent addition of discussion on various numerical modeling methods have improved the usefulness of this manuscript. However, the following still needs to be resolved prior to publication:

1. Issues surrounding Figure 7 (Comment 4 from previous review): Since it has been decided to present the data despite such variation in operating conditions (especially temperatures) and given the lack of visible trends in Figure 7, the authors should consider presenting them (along with their operating conditions) as tables instead of figure.
2. The paragraphing issue in Section 3 (Comment 3 from previous review): The flow of the discussions in sub-section 3.1 is still confusing. Discussions on cathode started in Line 247, transitioning into anode/OER discussions in Line 292, followed by a summary of cathode optimization from Line 304 along with Figure 8, which is followed by a continued discussion on cathodes until Line 385 when discussions on anode/OER resumed. The authors should discuss all cathode optimizations cited (using paragraphing to aid organization of ideas), followed by a summary and Figure 8, and then followed by discussions on anode/OER in another paragraph. Similar issues in other sub-sections should be rectified, if any.
3. The issue with the lack of integration of descriptions of figures with the relevant discussions in prior paragraphs (Comment 8 of the last review) is not resolved. For avoidance of doubt, integration of descriptions with the main text does not mean attaching a paragraph of descriptions to the previous paragraph and adding logical linkers. For example, the descriptions in Lines 462 - 474 for Figure 10b and Figure 10c are pertaining Reference 62, which should have been addressed in the relevant discussion in Lines 412 - 415. Another example being the descriptions of Figure 12a in Lines 699-703 pertaining Reference 93, which should have been discussed when Reference 93 is discussed in Lines 664 - 670. If such integration seems to be too awkward for any of the figures, it probably meant that the figure used does not aid the discussion in the main text and hence, should be removed.
4. Comment 14 from the previous review: It should have been "L CH₄/g_COD", "L_CH4/g_COD" or "L_methane/g_COD".
5. Relevant permissions for the reprinting/reusing of figures from published works in this manuscript should be obtained prior to the publication of this manuscript. A description of the obtained permission and a copyright statement should be included in the captions. (The authors should obtain advice/instructions from the Editor on this, if necessary.)

Author Response

We sincerely thank the Editor and Reviewers for their valuable time, constructive feedback, and thoughtful suggestions on our manuscript. We have carefully revised the manuscript in line with all recommendations. The modified sections are clearly highlighted in Yellow for ease of reference.

A detailed, point-by-point response to each comment from the Editor and Reviewers is provided below. We hope that the revised version meets the standards for publication in the “Eng”.

Reply to Reviewer 1

The revised manuscript has addressed many of the concerns highlighted in the previous round. The recent addition of discussion on various numerical modeling methods have improved the usefulness of this manuscript. However, the following still needs to be resolved prior to publication:

Comment 1: Issues surrounding Figure 7 (Comment 4 from previous review): Since it has been decided to present the data despite such variation in operating conditions (especially temperatures) and given the lack of visible trends in Figure 7, the authors should consider presenting them (along with their operating conditions) as tables instead of figure.

Reply 1: Thank you for the thoughtful suggestion. Upon re-evaluating the data behind Figure 7, we concluded that the variation in experimental conditions across studies and the lack of consistent trends made both the figure and a corresponding table scientifically unreliable. To preserve clarity and avoid possible misinterpretation, we have removed Figure 7 and the associated discussion in the revised manuscript.

Comment 2: The paragraphing issue in Section 3 (Comment 3 from previous review): The flow of the discussions in sub-section 3.1 is still confusing. Discussions on cathode started in Line 247, transitioning into anode/OER discussions in Line 292, followed by a summary of cathode optimization from Line 304 along with Figure 8, which is followed by a continued discussion on cathodes until Line 385 when discussions on anode/OER resumed. The authors should discuss all cathode optimizations cited (using paragraphing to aid organization of ideas), followed by a summary and Figure 8, and then followed by discussions on anode/OER in another paragraph. Similar issues in other sub-sections should be rectified, if any.

Reply 2: We sincerely thank the reviewer for this constructive and detailed comment. In the revised manuscript, we have comprehensively restructured Section 3.1 (AWE) to improve the logical flow and paragraphing as recommended.

Specifically:

• All discussions related to cathode optimization strategies (including Ni-based materials, nanostructured alloys, and surface-engineered cathodes) are now consolidated into a continuous and clearly demarcated section.
• This is followed by a summary paragraph and the accompanying Figure 7 (earlier Fig. 8), which synthesizes the cathode advancements in a structured mind map.
• The discussion on anode/OER materials is then introduced in a separate, dedicated paragraph, avoiding the intermixing seen in the earlier version.
• We have also reviewed and refined paragraphing in other related sections (e.g., PEM and SOEC) to maintain structural consistency and enhance readability throughout the manuscript.

We believe these changes address the reviewer’s concern fully and significantly improve the clarity and coherence of the technical discussion.

Comment 3: The issue with the lack of integration of descriptions of figures with the relevant discussions in prior paragraphs (Comment 8 of the last review) is not resolved. For avoidance of doubt, integration of descriptions with the main text does not mean attaching a paragraph of descriptions to the previous paragraph and adding logical linkers. For example, the descriptions in Lines 462 - 474 for Figure 10b and Figure 10c are pertaining Reference 62, which should have been addressed in the relevant discussion in Lines 412 - 415. Another example being the descriptions of Figure 12a in Lines 699-703 pertaining Reference 93, which should have been discussed when Reference 93 is discussed in Lines 664 - 670. If such integration seems to be too awkward for any of the figures, it probably meant that the figure used does not aid the discussion in the main text and hence, should be removed.

Reply 3: We thank the reviewer for highlighting this critical issue. In this revised version, we have ensured that all figures are now fully integrated into the technical discussion where the corresponding references are introduced. Specifically:

• The discussion of Tufa et al. [62] now includes the explanation of Figures 10(b) and (c) within the main paragraph.
• Similarly, the study by Kus et al. [93] is now accompanied by the discussion of Figure 11(a) directly where it is introduced.
• Figures 11(b–f) and Figure 12 are also described alongside their respective references in the PEMWE section.

Detached figure summaries previously placed after discussions have been removed to improve flow and clarity. This restructuring aligns with the reviewer’s recommendations and enhances the integration of visuals into the manuscript narrative.

Comment 4: Comment 14 from the previous review: It should have been "L CH4/gCOD", "LCH4/gCOD" or "Lmethane/gCOD".

Reply 4: Thank you for your comment. We have updated "LCH₄/gCOD" to "L CH₄/g_COD" as suggested for clarity.

Comment 5: Relevant permissions for the reprinting/reusing of figures from published works in this manuscript should be obtained prior to the publication of this manuscript. A description of the obtained permission and a copyright statement should be included in the captions. (The authors should obtain advice/instructions from the Editor on this, if necessary).

Reply 5: Thank you for your helpful comment. We have obtained the necessary permissions for the reprinting/reuse of figures from published works included in our manuscript. We have updated the figure captions to include the descriptions of the permissions, as required.

Author Response File: Author Response.docx