Impact of the Anode Serpentine Channel Depth on the Performance of a Methanol Electrolysis Cell

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This is a work that investigate the impact of the anode serpentine channel depth on the performance of a methanol electrolysis cell. Although the work is not entirely novel, such investigations are available. However more understanding of fuel flow in/through/over anodes is essential. The following observations needs to be addressed. 

1. The authors need to quantify percentage change in mass transfer efficiency, crossover methanol crossover, and polarization and power density in the abstract and conclusions.
2. What was accuracy in measurement of voltage, current density?
3. The authors need to improve Fig 2. The quality is poor and left side axis title is exceeding the limits.
4. The title of the graphs can be made smaller by putting critical information inside the graph as much as possible. This will help improved readability. 
5. In Fig 6 why overpotential is obtained in the ohmic region? Justify.
6. With regard to polarization curves, What is the current density of MEC and DMFC operating under similar conditions? How do authors compare and justify the results obtained for the present case?
7. From line 215 to 259, passive voice and past perfect tense should be adopted. Similarly typos at various places were observed. It is advised to check the manuscript thoroughly before submission of revised manuscript.
8. This Journal is inter-disciplinary in nature. Authors should add the direct scope and application of this work at the end of conclusion.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This study is the first to explore the effect of the depth of the anode serpentine channel on the performance of methanol electrolyzers (MECs) and direct methanol fuel cells (DMFCs). Core findings: Shallow channels enhance the mass transfer but intensify methanol permeation, while deep channels reduce permeation but hinder mass transfer. MEC produces more CO₂ due to high current density, and the optimal depth is 1.0 mm; DMFC is suitable for 0.5 mm channels. For the first time, the channel depth effect of the two devices was systematically compared. The experimental design was rigorous, revealing the core impact of the difference in CO₂ production on the optimal design. The shortcoming is that it only focuses on a single serpentine flow field and does not compare other flow field configurations; the conclusion is based on a small-sized single cell of 16 cm², and large-scale application needs further verification.

The content of the manuscript is within the scope of the journal and can be of broad interest to readers. However, in terms of specific content, there is still room for improvement. Therefore, I decided to give the decision of minor revision. It is recommended that the author properly absorb the reviewers' comments and make corresponding improvements and enhancements.

1. Page 1, 'The global push towards sustainable energy solutions has accelerated research into low-carbon energy carriers and efficient storage methods [2]. Hydrogen has emerged as a promising alternative fuel, valued for its clean use in fuel cells, producing only water as a byproduct during its operation [3,4].'
    I think the connection between renewable energy and hydrogen is not very clear and should be further clarified. Most of the renewable energy sources are intermittent, opening spatial and temporal gaps between the availability of the energy and its consumption by end users. To address these issues, it is necessary to develop suitable energy conversion systems for the power grid (10.1016/j.electacta.2019.03.056). This has undoubtedly promoted the rapid development of hydrogen electrolysis and fuel cell technology. 

2. The manuscript does not explain in depth the quantitative correlation mechanism between CO₂ bubble dynamics and mass transfer efficiency at different channel depths. It is recommended to supplement the theoretical model of gas-liquid two-phase flow to clarify the physical nature of deep channels suppressing bubble blockage.

3. The change in anode channel depth did not synchronously control the conservation of cross-sectional area, resulting in the difference in flow velocity and Reynolds number becoming a confounding variable. It is necessary to explain how to isolate the independent influence of the depth parameter; otherwise, the reliability of the conclusion is questionable.

4. Page 3,  'Despite the interest generated by DMFCs, there are still technical issues limiting their widespread commercialization. Among these limitations are the high cost of catalysts and membranes, slow reaction kinetics, or methanol crossover through the membranes [33,34].'
    It seems the degradation issues have never been considered as challenges. It should be noted that, fuel cells will experience a gradual decline in performance during long-term operation. This performance degradation is caused by a variety of complex factors, including the degradation of electrode materials, a loss of catalysts, mechanical damage to the membrane electrode assembly, and fluctuations in operating conditions. As reported in the degradation prediction of proton exchange membrane fuel cell performance based on a transformer model, performance degradation not only affects the efficiency and output power of fuel cells but also shortens their service life and increases maintenance and replacement costs.

5. Figure 1(b) only shows a photo of the anode plate at a depth of 0.5 mm, and does not provide a comparison of the 1.0/1.5 mm depth plate or a 3D structural diagram. It is strongly recommended to add multi-angle CAD drawings and key dimension annotations; otherwise, the consistency of the flow field structure cannot be verified.

6. The stainless steel 316L substrate is used, but the specific effect of surface treatment on contact resistance is not discussed. The initial slope data of the I-V curve needs to be supplemented. Especially for the high current density conditions of MEC, the interface resistance may become a performance-limiting factor. This omission affects the engineering value assessment.

7. The mass transfer resistance and activation loss at different depths were not quantified by electrochemical impedance spectroscopy (EIS). It is recommended to supplement the Nyquist plot and equivalent circuit fitting data; otherwise, the assertion of "mass transfer efficiency" lacks direct evidence.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf