Development of Membrane Electrode Assembly with Double-Catalytic Layer for Micro Direct Methanol Fuel Cell

Round 1

Reviewer 1 Report

This article is comprehensive, logically organized, and contains valuable information on the impact of the development of membrane electrode assembly with a double-catalytic layer for micro direct methanol fuel cells.

To improve the manuscript, the authors should consider the following modifications:

(1) The authors mentioned that a slurry containing XC-72, PTFE (20 wt.%) and Nafion (10 wt.%) was coated onto carbon paper. What is the particle size of PTFE material? What is the molecular weight of Nafion material? It is noted that the authors should prepare a gas diffusion layer (GDL) with various particle sizes of PTFE material and various concentrations of Nafion solution for the preparation of gas diffusion electrode (GDE) and their performance for the coherence of the subject.

(2) The authors prepared the catalyst-coated membrane (CCM) using commercial Nafion 117 membranes. It is noted that the thickness of the Nafion 115, Nafion 117, and Nafion 1110  are 125, 183, and 254 micrometers.  During the coating process,  Nafion boiling at 80℃ expands to a state with 19-22 H₂O molecules per –SO₃H group. Upon water take-up, the hydrophilic center expands in size, shaping an organization of water channels with a diameter of around 5 nm, which can differ along the length of a given channel. This organization permits quick vehicle of protons and polar particles, water, and methanol, across the Nafion membrane. The authors should compare the CCM performance using Nafion 115, and Nafion 1110 as well to gain a better understanding of the cluster-network model.

The submitted manuscript has significant scientific insights and the conclusions are soundly supported by the experimental and modeling data. However, However, the manuscript requires minor revisions before being accepted in the Special Issue: Innovative Approaches for Fabricating Membrane Electrode Assemblies for Fuel Cells and Water Electrolysis Applications in a well-circulated journal, Inventions.

Author Response

Dear reviewer:

Thank you very much for taking the time to review our manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted in the re-submitted manuscript. (Figures are submitted in the PDF file)

1. As suggested, the manuscript is now with a much more detailed description of the materials. The particle size of PTFE material is 120 nm. The molecular weight of Nafion material is about 544. The following figure shows that the particle size of PTFE material has an impact on the DMFC performance. A larger particle size of PTFE, 3~4 μm, could just ruin the performance of a DMFC due to obvious non-uniform distribution in porous materials and high chances of blocking micropores. Based on the experimental results, we find smaller PTFE particle size is better. In addition, to the best of our knowledge, the amount of PTFE material as well as the Nafion added into the catalysts should be limited to a very low level. However, a deep insight and full study into this topic is beyond the scope of this paper, so we didn’t include these experimental results which cannot directly support our point of view into the manuscript.

    

2. From the perspective of engineering application, Nafion 1110 is not a good choice for PEMFCs because of its high resistance to Ion conduct. Here we illustrate a simple CCM performance comparison between Nafion 115, and Nafion 1110. The result reveals that membrane thickness matters, but it is more of a topic on the Electronic-drag Mechanism and methanol crossover than our current topic. The results didn’t help us to expand and strengthen our study on the effects of different membranes. Therefore, in our daily researches, Nafion 1110 is not commonly used and not suggested in PEMFC applications.

Author Response File: Author Response.pdf

Reviewer 2 Report

Summary

The authors propose a novel methanol fuel cell with dual-layer catalysts, the catalyst in the outer layers being carbon supported whereas in the inner layer unsupported. Furthermore, the study employs computational simulation as well as lab scale experiments indicating the advantages of the proposed structure in terms of uniformity of the resulting current density and reduced methanol crossover.

General comments

The authors have embarked on an ambitious endeavor with a resourceful study, the topic of which, involving an improved structure for a methanol fuel cell, is suitable for the Inventions journal. Moreover, the sectional structure of the manuscript is appropriate for a scientific article. I however have a couple of comments regarding the mathematical aspects of the study.

1.       The description of the use of the COMSOL software is rather brief. Did you perform a simultaneous simulation of all the coupled differential equations in one big model or was it possible to break the task into smaller parts?

2.       Did you use some of the specific physics modules of COMSOL or just the mathematics module?

3.       According to the manuscript, the model is two-dimensional. However, the need for more than one dimension in the model is not obvious. In fact, all the visualizations of the simulations in the Results section seem to be essentially one-dimensional.

4.       Would it be possible to compare the simulations with some experimental results?  

The table is well organized, and the figures are of good quality. In addition, the typography of the equations is also adequate.

Specific comment

Line 126: But does not Darcy’s law already decide the velocity? To me it seems that Equation (12) decides the concentration.

It is not easy to find any grammatical errors in the text, but at times the choices of word and formulation are a bit surprising.

Line 107: The driving force…

Line 113: The initial word in the sentence would suggest that there is some difference. Analogously with would be better in my opinion. Furthermore, the word “following” seems rather redundant.

Line 130: This simplification has little effect on the relevance of the following discussion.

Author Response

Dear reviewer:

Thank you very much for taking the time to review our manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted in the re-submitted manuscript.

1. We always perform the simulation of all the coupled differential equations in one big model. To the best of our knowledge, we can only get an extreme or unilateral solution if we don’t think through the Multi-physics in the DMFC. So, the model should include all the coupled differential equations as we can. However, when the simulation gets too complex and complicated, appropriate assumptions and simplifications are needed.
2. We use the specific physics modules including Transport of Diluted Species, Porous Media, Electric currents, multiphase flow, chemistry reaction, as well as some self-defined equations in the mathematics module.
3. Now, the model is revised to show its 2D structure.
4. Figure 7 is the experimental results which can validate the main simulation result shown in Figure 6.

In addition, the Quality of English Language is improved as we fined down the choices of word and formulation in the lines of 107, 113, 130, etc.

Reviewer 3 Report

Ms No: Inventions – 2809074

Title: Development of membrane electrode assembly with double-catalytic layer for micro direct methanol fuel cell

Authors: Shubin Zhang, Yanfeng Jiang

Evaluation

The present paper deals with the development of a double catalytic layer which improves the performance of direct methanol fuel cells. Towards this direction authors made measurements and mathematical modeling in order to study the effect on the DMFC’s performance. It contains plenty useful technical information. However, the paper requires some major changes in order to be published in the Inventions journal, based on the comments below. 

Comments

1.The introduction requires enrichment in order to provide the potential reader more information as it concerns the state-of-the-art catalytic materials applied in direct methanol fuel cells. For instance, a short paragraph should be added containing the most recent research efforts towards the development of highly active catalytic materials for both the anode and the cathode of DMFCs. Moreover, the state-of-the-art catalysts should be mentioned. Are they different from the state-of-the-art catalysts of PEMFCs? Are they similar and why? All these information would facilitate the potential reader to comprehend the frame of the design of electrocatalytic materials for DMFCs and this would furtherly reinforce the significance of the present work.

2.Authors should mention the main reactions occurring in the DMFC. They should also add a relevant text mentioning which of the two main reactions is slower and why. And also provide relevant references.

3.If possible, the quality of Figures 1-5 should be improved. For instance, the “x10⁸” should be more separated from the numbers.  

4. As it concerns the commercial catalysts used for the DMFC measurements, is there any information for the carbon support applied? Is it carbon black or activated carbon or something else? If these materials are physicochemically characterized by the company authors should provide additional information. If this information are not given by the company is there a chance that authors could perform a rapid physicochemical characterization implementing BET and XRD methods in order to estimate the specific surface area and the structural characteristics of the electrocatalytic powders. This information may attract researchers with heterogeneous catalysis and electrochemistry backgrounds, since this information are strongly related to the variations of the cell performance.

5.If instead of isopropyl alcohol, isopropanol or ethanol was applied, this would make any difference in the deposition of the electrocatalyst on the carbon cloth?

6.Authors should mention the noble metal loading of anode and cathode in order to help the potential reader in comparing the results of the present work with other relevant works in literature. I noticed that in the results section authors stated that “with a total metal loading of 4 mg cm^-2 on the anode and 2 mg cm^-2 on the cathode”. Authors should note the exact wt.% noble metal loading of the anode and the cathode in the experimental section. The highest metal loading of the anode is attributed to the sluggish kinetics of the anodic methanol oxidation reaction? Authors should comment on this and add a relevant text in the manuscript.

7.Please provide relevant references in the experimental section.

8.Authors should enrich the discussion section with comparison with literature. The results of the present study are very intriguing and they should be compared with those already published (the most recent) otherwise their significance is reduced. Please make an effort on this.

9.If possible, could the authors try to normalize the resulting DMFC power with the noble metal loading? Please comment on this.

Comments for author File: Comments.pdf

Author Response

Dear reviewer:

Thank you very much for taking the time to review our manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted in the re-submitted manuscript.

1. The introduction is enriched. A short paragraph containing the most recent research efforts towards state-of-the-art catalytic materials applied in DMFCs is added.
2. The reactions occurring in the DMFC is now fully introduced.
3. The quality of the Figures 1-5 is improved with clearer numbers.
4. Details of the physicochemical characteristics of the commercial catalysts are now indicated in the Material Section.
5. To deposit the electrocatalyst on the carbon cloth, isopropanol can be used but ethanol is not a good idea (active for direct ignition or CO-related catalyst poison). We revised the Method Section to make the experimental procedure clearer.
6. We added a relevant text in the manuscript indicating the noble metal loadings of the anode and the cathode.
7. Relevant reference is added in the experimental section.
8. Comparison with literature is added in the manuscript.
9. Although the performance of each MEA we made is not normalized, the noble metal loadings are kept at constant values, namely 4 mg cm-2 for the anode and 2 mg cm-2 for the cathode. So, we added normalized DMFC performance with the noble metal loading in the text to make this information clearer.

Round 2

Reviewer 3 Report

Authors made all the appropriate changes based on the suggestions and the resubmitted paper is clearly improved. According to my opinion the paper in its present form is suitable for publication in Inventions journal.