Proton Exchange Membrane Electrolysis Revisited: Advancements, Challenges, and Two-Phase Transport Insights in Materials and Modelling

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

General comments: The paper reads well. Most of the information are available in literature. However - adding details about models has provided value to PEMEC research. My comments are below

1. The abstract  of the paper should be concise (≤300 words) and must emphasize key findings.
2. The body of texts contains redundant information—rewrite the paper on the most critical insights such as modelling or current issues
3. Section 1: Currently it is too long and most of the details are available in the textbook.
4. Replace the section 1 with a concise table which must summarise key concepts
5. Equations: Equations 4 and 5 must explicitly include cell potential terms. Present all equations in differential form and avoid nabla notation.
6. Ensure equation numbers are correctly formatted.
7. Content Focus: The review should critically assess published studies rather than simply summarizing the findings.
8. Kindly Remove authors' own results—this is a review, not a research article.
9. Focus on the modelling aspects and eliminate unnecessary details
10. References: Include recent publications relevant to this research area. Ensure complete citation details.
11. Language and Formatting: Fix typos and ensure clarity.
12. Remove repetitive content across sections.
13. Add the contents related to the following references

Østenstad, J. (2023). Multiphysical modeling of a next generation PEM electrolyser.

Tofighi-Milani M, Fattaheian-Dehkordi S, Lehtonen M. Electrolysers: A Review on Trends, Electrical Modeling, and Their Dynamic Responses. IEEE Access. 2025 

 

 

Comments on the Quality of English Language

Needs little improvement

Author Response

R1.C0: General comments: The paper reads well. Most of the information are available in literature. However, adding details about models has provided value to PEMEC research. My comments are below:

Response:

We sincerely appreciate the reviewer’s thoughtful feedback and their recognition of the value added by our discussion on modeling in PEMEC research. We acknowledge the points raised and will carefully address all comments to improve the clarity, conciseness, and critical assessment of the manuscript. Thank you for your valuable insights, which have guided us in enhancing the quality of this review.

R1.C1: The abstract of the paper should be concise (≤300 words) and must emphasize key findings.

Response:

We appreciate the reviewer’s suggestion to refine the abstract for conciseness while emphasizing key findings. In response, we have restructured and condensed the abstract to 298 words while ensuring it highlights the most critical advancements, challenges, and modeling insights in PEMEC technology. The revised abstract now provides a clearer focus on numerical modeling, material improvements, and system integration with renewable energy sources.

We believe these changes enhance clarity and increase the manuscript's overall impact. Thank you for your valuable feedback. The following is the revised abstract: (see page 1)

The transition to clean energy has accelerated the pursuit of hydrogen as a sustainable fuel. Among various production methods, Proton Exchange Membrane Electrolysis Cells (PEMECs) stand out due to their ability to generate ultra-pure hydrogen with efficiencies exceeding 80% and current densities reaching 2 A/cm². Their compact design and rapid response to dynamic energy inputs make them ideal for integration with renewable energy sources. This review provides a comprehensive assessment of PEMEC technology, covering key internal components, system configurations, and efficiency improvements. The role of catalyst optimization, membrane advancements, and electrode architectures in enhancing performance is critically analyzed. Additionally, we examine state-of-the-art numerical modelling, comparing zero-dimensional to three-dimensional simulations and single-phase to two-phase flow dynamics. The impact of oxygen evolution and bubble dynamics on mass transport and performance is highlighted. Recent studies indicate that optimized electrode architectures can enhance mass transport efficiency by up to 20%, significantly improving PEMEC operation. Advancements in two-phase flow simulations are crucial for capturing multiphase transport effects, such as phase separation, electrolyte transport, and membrane hydration. However, challenges persist, including high catalyst costs, durability concerns, and scalable system designs. To address these, this review explores non-precious metal catalysts, nanostructured membranes, and machine-learning-assisted simulations, which have demonstrated cost reductions of up to 50% while maintaining electrochemical performance. Future research should integrate experimental validation with computational modelling to improve predictive accuracy and real-world performance. Addressing system control strategies for stable PEMEC operation under variable renewable energy conditions is essential for large-scale deployment. This review serves as a roadmap for future research, guiding the development of more efficient, durable, and economically viable PEM electrolyzers for green hydrogen production.

R1.C2: The body of texts contains redundant information—rewrite the paper on the most critical insights such as modelling or current issues.

Response: We sincerely appreciate the reviewer’s valuable feedback. In response, we conducted a comprehensive review of the entire manuscript to identify and eliminate redundant content and repetitive phrasing. Our goal was to ensure a more focused and coherent narrative that emphasizes the most critical aspects of the field - particularly modeling approaches, recent challenges, and emerging trends in PEMEC research.

We have significantly condensed and streamlined Sections 1 and 2, removed overlapping explanations in later sections, and refined the discussion of component-level strategies to maintain relevance to modeling and performance optimization. These efforts ensure the revised manuscript is more concise, reader-friendly, and directly aligned with the review’s modeling-centered focus.

R1.C3: Section 1: Currently it is too long and most of the details are available in the textbook.

Response:

We appreciate the reviewer’s observation. Although we believe the original Section 1 served as a focused introduction with limited textbook-level content, we have revised it to remove any redundant or non-essential details. The section has been carefully streamlined to maintain its purpose while improving clarity. Additionally, since Section 2 contained more textbook-derived information, we have also addressed that section by significantly condensing the content and summarizing it in tables to align with the reviewer’s intent.

R1.C4: Replace the section 1 with a concise table which must summarize key concepts

Response:

Thank you for the helpful suggestion. While we believe Section 1 was concise in its original form, we have still revised it to remove any unnecessary content. In parallel, we have also restructured Section 2—which contains more detailed, textbook-like information—by summarizing its key content into tables as recommended. This ensures both sections are now more concise and accessible.

R1.C5: Equations 4 and 5 must explicitly include cell potential terms. Present all equations in differential form and avoid noble notation.

Response:

We thank the reviewer for the helpful suggestion. In response, Equation (4) has been revised to explicitly include the standard cell potential, with values assigned to the anode and cathode reactions, and the net cell voltage clarified. Equation (5) has also been rewritten in differential form and reformatted as a curly bracket system to clearly represent the molar production and consumption rates of water, hydrogen, and oxygen. Furthermore, we clarified that the current density iii, appearing in these expressions, is governed by electrochemical kinetics and is later related to the cell potential through the Butler–Volmer equation. Noble notation has not been used, as per the reviewer’s recommendation.

R1.C6: Ensure equation numbers are correctly formatted.

Response:

We thank the reviewer for pointing this out. All equation numbers throughout the manuscript have now been reviewed and reformatted to ensure consistency and proper placement. This includes alignment, numbering style, and reference formatting to maintain clarity and professionalism.

R1.C7: Content Focus: The review should critically assess published studies rather than simply summarizing the findings.

Response:

We thank the reviewer for this insightful comment. In response, we have revised several sections (notably Sections 3, 4) to shift the tone from descriptive to analytical. Specifically, we now provide critical assessments of published modelling and experimental studies, highlighting their assumptions, limitations, and relative strengths. These changes enhance the paper’s analytical depth and align with the core expectations of a review article.

R1.C8: Kindly Remove authors' own results—this is a review, not a research article.

Response:

We thank the reviewer for this important clarification. In response, we have carefully removed all results originating from our own numerical simulations, including associated figures (previously labeled as Figs. 8a, 8b, 9a, 10a, and 11), their captions, and all related discussion in the main text. Figure 11 has also been revised to exclude our own data and now features results entirely sourced from published literature. All figures included in the revised manuscript now reflect previously published results from other studies, such as those by Nie et al. [95, 96], with proper citations and permissions from Elsevier where applicable. We have ensured that all analysis and commentary are based solely on external sources, fully aligning the revised manuscript with the expectations of a review article.

R1.C9: Focus on the modelling aspects and eliminate unnecessary details

Response:

We sincerely thank the reviewer for this insightful suggestion. In response, we have carefully revised the manuscript to improve focus on modeling aspects, while removing descriptive or repetitive details that do not contribute directly to the core objectives of the review.

• Specifically, Section 3 (Advantages and Challenges of PEMEC) has been rewritten to emphasize how PEMEC-specific operational complexities (e.g., degradation under dynamic loads, gas-liquid interactions) necessitate advanced modelling approaches such as two-phase flow, transient simulations, and multiphysics coupling. Redundant general information has been removed, and the discussion is now framed around modelling relevance and current limitations in the literature.
• In Section 4.3, we have retained the essential technical background for performance improvement strategies (catalyst, membrane, electrode design), but condensed overly descriptive parts. More importantly, we have added critical assessments of existing experimental studies, highlighting where modelling gaps exist—for example, the lack of real-geometry flow-field simulations, inadequate representation of pore-scale transport in multilayer electrodes, and underdeveloped models for membrane swelling or degradation.

R1.C10: References: Include recent publications relevant to this research area. Ensure complete citation details.

Response:

We appreciate the reviewer’s reminder regarding the inclusion of recent and relevant publications. In response, we have added several up-to-date references published in 2023 and 2024 that reflect recent advances in PEMEC modelling, materials, and system-level optimization. In particular, the following key studies have been incorporated:

• Østenstad, J. (2023). Metaphysical modelling of a next generation PEM electrolyser.
• Tofighi-Milani M., Fattaheian-Dehkordi S., Lehtonen M. (2025). Electrolysers: A Review on Trends, Electrical Modeling, and Their Dynamic Responses. IEEE Access.

We also conducted a thorough check to ensure that all references include complete citation details, including author names, titles, journal names, volume/issue numbers, and publication years. Where available, we have also added DOIs for consistency and accuracy.

These updates ensure that the manuscript is better aligned with the current state of the field and provides readers with a more comprehensive view of recent contributions.

R1.C11: Language and Formatting: Fix typos and ensure clarity.

Response:

We thank the reviewer for highlighting the importance of language clarity and formatting consistency. In response to this comment, we have undertaken a careful proofreading of the entire manuscript to address typographical errors, improve sentence structure, and enhance overall readability.

• Numerous sentences have been revised for clarity, conciseness, and academic tone, especially in technically dense sections.
• We also ensured uniform formatting of headings, subheadings, tables, and references, adhering closely to the journal’s formatting guidelines.
• Where necessary, complex descriptions have been rephrased to avoid ambiguity and improve flow for interdisciplinary readers.

We believe these revisions have significantly improved the quality and presentation of the manuscript, and we appreciate the reviewer’s attention to this important aspect.

R1.C12: Remove repetitive content across sections

Response:

We thank the reviewer for pointing this out. As also mentioned in response to earlier comments (R1.C2 and R1.C9), we have thoroughly reviewed the entire manuscript to identify and remove repetitive and redundant content. This includes eliminating overlapping explanations across sections, condensing repeated background information, and ensuring that each section contributes unique, focused content.

These revisions were made with the goal of improving clarity, flow, and alignment with the paper’s modelling-oriented focus. We believe the manuscript is now more concise and streamlined without losing technical depth.

R1.C13: Add the contents related to the following references:

• Østenstad, J. (2023). Multiphysical modelling of a next generation PEM electrolyser.
• Tofighi-Milani M, Fattaheian-Dehkordi S, Lehtonen M. Electrolysers: A Review on Trends, Electrical Modeling, and Their Dynamic Responses. IEEE Access. 2025

 Response:

We thank the reviewer for suggesting the inclusion of the above references. As mentioned in our responses to earlier comments (R1.C2, R1.C7, and R1.C9), we have carefully reviewed these works and incorporated their content into Sections 4.3.2 and 4.3.3. The cited material supports our critical assessment of modelling challenges, dynamic system behavior, and multiphysics coupling strategies in PEMEC design. These additions have improved the technical depth of the review and aligned the manuscript with recent advancements in the field.

Full references have been added to the reference list.

Reviewer 2 Report

Comments and Suggestions for Authors

In this manuscript, the advancements, challenges, and two-phase transport insights in materials of PEMECs were comprehensively reviewed. The following comments can be considered.

1Try not to cite too many refs in the same place, such as [13-26].

2 Section 2.1Alkaline water electrolysis should be alkaline water electrolysis cells. Please revise the title of section 2.2 and 2.3.

3More related studies about the flow field design in the BPs can be added in the section 4.

4.The parameters of PTLs and CLs can be added, such as tortuosity, electric conductivity.

5.Please check the eq.11.

6 A summary of the model and software used by the researchers can be added as a Table.

7.Only the source terms of energy equation are presented in Table 7. The source terms of the governing equations can be added.

8.The conclusion section can be extended to include more information about future research direction.

Author Response

R2.C0: In this manuscript, the advancements, challenges, and two-phase transport insights in materials of PEMECs were comprehensively reviewed. The following comments can be considered.

 Response:

We sincerely thank Reviewer 2 for the thoughtful assessment of our manuscript and for recognizing its contribution in reviewing advancements, challenges, and two-phase transport insights in PEMEC materials. We appreciate the constructive comments and have addressed each of them carefully as detailed below.

 R2.C1: Try not to cite too many refs in the same place, such as [13-26].

 Response:

We appreciate the reviewer’s observation. In response, we have thoroughly reviewed all instances in the manuscript where large reference blocks appeared (e.g., [13–26]) and reduced citation density by:

• Keeping only the most relevant or representative references in grouped citations.
• Distributing citations more contextually across the text, where appropriate, to improve clarity and avoid clutter.
• Ensuring that citation formatting remains consistent and concise throughout.

R2.C2:  Section 2.1Alkaline water electrolysis should be alkaline water electrolysis cells. Please revise the title of section 2.2 and 2.3.

Response:

We appreciate the reviewer’s observation regarding the accuracy and consistency of section titles. However, based on a critical comment from Reviewer 1 (R1.C3 and R1.C4), we significantly revised Section 2 by condensing its content into a few focused paragraphs and two summary tables, and removed all subsections (2.1–2.3) to streamline the manuscript and reduce textbook-like material.

That said, we fully acknowledged Reviewer 2’s suggestion and ensured that in the revised version, the terminology “Alkaline Water Electrolysis Cells” is used wherever relevant, in alignment with the reviewer’s recommended phrasing.

R2.C3: More related studies about the flow field design in the BPs can be added in the section 4.

Response:

We thank the reviewer for this valuable suggestion. In response, and also in alignment with comments from Reviewer 1 (R1.C2, R1.C7, and R1.C9), we have significantly improved the discussion on flow-field design in Section 4.3.3.b by:

• Adding insights from recent studies, including those by Østenstad (2023) and Tofighi-Milani et al. (2025), which address flow-field geometry effects and the need for advanced modelling of pressure profiles and transient dynamics.
• Emphasizing the importance of integrating real 3D flow-field geometries into simulations, especially in the context of two-phase behavior and system-scale performance.
• Highlighting the limitations of current modelling approaches, which often rely on simplified or idealized flow-field assumptions, and advocating for more comprehensive frameworks that couple fluid flow, electrochemical reactions, and phase transport.

These updates not only respond to this comment but also strengthen the manuscript’s overall modelling focus and critical assessment, as recommended by both reviewers.

R2.C4: The parameters of PTLs and CLs can be added, such as tortuosity, electric conductivity.

Response:

We thank the reviewer for this insightful comment. In response, we have expanded Table 6 to include additional relevant parameters for PTLs and CLs—specifically, tortuosity and electrical conductivity—which play critical roles in characterizing transport and electrochemical performance in porous media. These additions improve the technical depth of the manuscript and further support modelling considerations.

R2.C5: Please check the eq.11.

Response:

We thank the reviewer for pointing this out. The issue with Equation 11 appears to have been caused by text distortion during the transfer of the manuscript into the MDPI template. We have carefully reviewed the equation and confirm that it is mathematically correct. The formatting issue has now been resolved in the revised manuscript.

R2.C6:  A summary of the model and software used by the researchers can be added as a Table.

Response:

We thank the reviewer for this helpful suggestion. In response, we have added a new table (Table 10) summarizing selected PEMEC modelling studies, including the software platforms, model types, and key remarks. This addition improves the clarity of the modelling discussion and highlights the diversity of simulation approaches used in the field.

To ensure smooth integration, we also inserted a transition paragraph after Figure 6, introducing the table as follows:

“To complement the discussion above, Table 10 provides a summary of selected PEMEC simulation studies, highlighting the modelling approaches, software platforms, and notable features used in the literature.”

This addition supports the technical depth of the manuscript and directly responds to comments regarding modelling focus and literature coverage.

R2.C7: Only the source terms of energy equation are presented in Table 7. The source terms of the governing equations can be added.

Response:

We thank the reviewer for this important observation. While the source terms were previously emphasized only in Table 7 (now Table 8), we have carefully revised the manuscript to ensure that all relevant governing equations—mass, momentum, species, proton/electron transport, and energy—now explicitly include their respective source terms in the equation definitions and accompanying explanations.

For example:

• The mass conservation equation (Eq. 12) includes the source term
• The momentum equation (Eq. 13) reflects S_m and the drag force is defined in Eq. 15,
• Species transport equations (Eq. 19) include the source term R_j
• Electron and proton potential equations (Eqs. 22–23) include ​​
• And the energy equation (Eq. 25) remains supported by Table 8, with clearly defined S_T

We believe this structure provides a comprehensive and integrated presentation of the source terms within the modelling framework, without redundancy. The formulation has been double-checked for consistency and clarity in the current revision.

R2.C8: The conclusion section can be extended to include more information about future research direction.

Response:

We appreciate the reviewer’s valuable suggestion. In response, we have revised and extended the conclusion section by incorporating additional forward-looking insights that highlight specific future research opportunities. In particular, we now emphasize the importance of establishing standardized validation frameworks, incorporating in-situ diagnostics, and exploring smart control strategies for PEMECs integrated with renewable energy systems. These additions build upon the core findings of the review and present a clearer roadmap for advancing the field. We have ensured that the added content flows naturally with the existing narrative while providing a more robust outlook on future development priorities.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Authors have addressed most of my comments. Yet - manuscript could be refined towards citation metrics and scores. I will leave the editor and authors to decide on this

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript has already been revised according to the comments. No further comments.