Evaluating the Effect of Metal Bipolar Plate Coating on the Performance of Proton Exchange Membrane Fuel Cells

Round 1

Reviewer 1 Report

Split the Introduction in 1) Introduction and 2) Materials for bipolar plates.

Extend the Introduction (for example, discuss the main challenging objectives for next decades, the main limitations in extensive commercialization, the limits of electrical and mechanical stresses etc.) and add at the end the main objectives and structure of the paper.

Author Response

Reviewer 1 Comments:

Split the introduction in 1) Introduction and 2) Materials for bipolar plates

Extend the Introduction (for example, discuss the main challenging objectives for next decades, the main limitations in extensive commercialization, the limits of electrical and mechanical stresses etc.) and add at the end the main objectives and structure of the paper

Response: The author’s appreciates the suggestions raised with regards to the extension of the introduction in relation to the different limiting aspects of commercialisation of the fuel cell. This article focused primarily on the coating of the bipolar plate in relation to the performance of the PEM fuel cell. The aforementioned suggestions will be considered in subsequent publication.

Action taken: The introduction has now been split into sections as advised. Also the introduction has been extended as suggested by the reviewer.

The introduction has been modified and the corresponding changes have been marked in yellow in the updated manuscript.

Reviewer 2 Report

This manuscript demonstrates an extensive review of various types of coating on metallic bipolar plates for proton exchange membrane fuel cells (PEMFCs). Appropriate coating techniques to enhance corrosion resistance and electrical conductivity are well written. However, the manuscript should be revised according to the following comments before being published. (1) The hydrophobic and hydrophilic properties of the bipolar plate would influence the ability to discharge water in the flow channels. The authors demonstrate that hydrophobic and hydrophilic properties are varied with the coating on the metallic bipolar plate as shown in Figures 6 and 13. The authors had better describe the effect of hydrophobic and hydrophilic properties on the cell performance.

Author Response

Reviewer 2 Comments:

This manuscript demonstrates an extensive review of various types of coating on metallic bipolar plates for proton exchange membrane fuel cells (PEMFCs). Appropriate coating techniques to enhance corrosion resistance and electrical conductivity are well written. However, the manuscript should be revised according to the following comments before being published. (1) The hydrophobic and hydrophilic properties of the bipolar plate would influence the ability to discharge water in the flow channels. The authors demonstrate that hydrophobic and hydrophilic properties are varied with the coating on the metallic bipolar plate as shown in Figures 6 and 13. The authors had better describe the effect of hydrophobic and hydrophilic properties on the cell performance.

Response: The hydrophobic and hydrophilic properties on the cell performance  originates from the contact angle hence, the authors explain the effect on the overall performance on the fuel cell using the Nyquist and bode plots as shown in Figure 7 and 8 in the updated manuscript.

Action taken: The manuscript has been updated and Figure 7 and 8 has been added to support the raised comment. The changes are marked in yellow on line 417 – 446        

Reviewer 3 Report

This manuscript reviewed research in PEMFC bipolar plate coating. I have the following comments.

I suggest the authors add “metal bipolar plate” before “coating” in the title to be more specific.

In Abstract

Line 18, what do the authors mean by strength?

What does the “interfacial contact resistance” mean? Is there any kinds of contact or contact resistance that is not interfacial?

In Introduction

The authors first claimed that “PEM fuel cells are … fuels” (Line 40-42), and then “PEM fuel cells convert electrochemical reaction … into electrical energy” (Line 42-43). This expressions are quite misleading. The authors could refer to some electrochemical literature in fuel cell direction (such as https://doi.org/10.1016/j.ijhydene.2010.07.055, and https://doi.org/10.1016/j.ijhydene.2012.07.043) to gain a clearer definite of PEMFC.

If as the authors said a PEMFC use H2 as fuel, then the practical voltage of the PEMFC is higher than 0.6-0.7 V. If as the authors said the practical voltage of a PEMFC is 0.6-0.7 V voltage, this PEMFC is likely to be a direct methanol PEMFC, which does not use hydrogen as fuel. Please check the consistence to avoid confusion.

Table 1, could the authors explain why the DOE 2020 targets for weight and H₂ permeation rate is higher than the 2017 targets?

Please clarify what do the “this” and “they” mean in the beginning of the paragraphs in Line 69 and 87, respectively.

In Line 193, please clarify what is “filer”.

Can the authors briefly explain why the contact resistance of polymer composite bipolar is low, considering that its electrical conductivity is low?

In Table 3, please clarify the meaning of “quick cycle time”.

In Section 4, a new literature on the impact of contaminant ions on polymer electrolyte membrane resistance and its mechanism can be found is https://doi.org/10.1016/j.memsci.2018.02.050.

 

Author Response

Reviewer 3 Comments:

This manuscript reviewed research in PEMFC bipolar plate coating. I have the following comments.

I suggest the authors add “metal bipolar plate” before “coating” in the title to be more specific.

In Abstract

Response: The changes suggested by the reviewer have been updated in the manuscript.

 

Comment: Line 18, what do the authors mean by strength?

Response: The updated manuscript has been modified to explain the exact material property (mechanical strength) in line 19 of the updated manuscript marked in yellow.

 

 

Comment: What does the “interfacial contact resistance” mean? Is there any kinds of contact or contact resistance that is not interfacial?

 

Response:  In PEM Fuel Cells the “Interfacial contact resistance’’ is the contact resistance at the interface of the bipolar plate and gas diffusion layer which is also considered as a characteristic parameter for fuel cells (Colleen, 2012, F. Barbir, 2005, Colleen, 2008).

 

Comment: The authors first claimed that “PEM fuel cells are … fuels” (Line 40-42), and then “PEM fuel cells convert electrochemical reaction … into electrical energy” (Line 42-43). This expressions are quite misleading. The authors could refer to some electrochemical literature in fuel cell direction (such as https://doi.org/10.1016/j.ijhydene.2010.07.055, and https://doi.org/10.1016/j.ijhydene.2012.07.043) to gain a clearer definite of PEMFC.

 

Response: The ambiguity in the earlier explanations of fuel cell has been clarified in the updated manuscript and referenced accordingly as shown in line 43-48 marked in yellow

 

Comment: If as the authors said a PEMFC use H2 as fuel, then the practical voltage of the PEMFC is higher than 0.6-0.7 V. If as the authors said the practical voltage of a PEMFC is 0.6-0.7 V voltage, this PEMFC is likely to be a direct methanol PEMFC, which does not use hydrogen as fuel. Please check the consistence to avoid confusion.

 

Response: The authors acknowledge the suggestion raised by the reviewer and the updated manuscript has been modified to capture the exact open circuit voltage for PEM fuel cell in line 52 marked in yellow

 

Comment: Table 1, could the authors explain why the DOE 2020 targets for weight and H2 permeation rate are higher than the 2017 targets?

 

Response:  It is observed that the DOE target for 2020 is higher compared to 2017. The goal is to improve the overall characteristics of materials used in the manufacturing of bipolar plates and since weight of the material is directly proportional to the cost, the research community anticipates that strictly adhering to these standards set by the U.S Department of Energy will reduce the cost of fuel cell by 2020 hence increasing their patronage as well as their application for commercial purposes. This will further enhance their competition with existing form of energy generation mediums like fossil products. An increase in hydrogen gas permeation indicates an increase in electrochemical reaction as well as current being generated from the cell.

 

Comment: Please clarify what do the “this” and “they” mean in the beginning of the paragraphs in Line 69 and 87, respectively.

 

Response: The authors have clarified the meaning of ‘’this’’ and ‘’they’’ in the updated manuscript in line 71 and 98 marked in yellow.

 

Comment:  In Line 193, please clarify what is “filer”.

Response:  The authors have corrected the spelling of “filer” to “filler” in the updated manuscript.

 

Comment: Can the authors briefly explain why the contact resistance of polymer composite bipolar is low, considering that its electrical conductivity is low?

 

Response: Anti-static materials are made up of polymer composites that are electrically conductive. Combining fillers and polymers is one of the methods of producing conductive polymer composites. Fuel cell bipolar plates require materials with high electrical conductivity. Understanding composite conductive characteristics is best explained from the percolation theory. The theory explains that insulating to conductive transition of the composites is due to formation of conductive path ways by the conducting fillers. Percolation threshold is therefore the amount of fillers needed for transition to occur. Increasing the fillers is the well-known strategy for increasing the electrical conductivity. A research work showed that 7500 micro sized graphite is required to produce composite with an electrical conductivity of 104s/m, which is suitable for bipolar plate in fuel cell. It must be noted that an increase in the filler has some negative implications as well. It affects the mechanical integrity, manufacturability as well as the cost of the conductive composite. This therefore explains that even though the contact resistance of the polymer composite is low, the filler content affects the electrical conductivity. Since most manufacturers consider profit during production, the content of the filler is generally reduced.

 

Comment: In Table 3, please clarify the meaning of “quick cycle time”.

 

Response:  Any manufacturing process needs to have optimised productivity and quality. In injection moulding of plastics, attention should be given on by part design, mould design and mould precision. This ensures productivity on account of zero defect moulding without rejection and optimised cycle time. Cycle-time optimisation starts at the design stage. Cooling time takes up over 50% of cycle time. Therefore understanding of cooling in the mould is very important. In summary quick cycle time simple implies the manufacturing time.

 

Comment: In Section 4, a new literature on the impact of contaminant ions on polymer electrolyte membrane resistance and its mechanism can be found is https://doi.org/10.1016/j.memsci.2018.02.050.

 

Response: The authors have considered this reference and made the necessary modification in the updated manuscript.

 

Round 2

Reviewer 1 Report

the authors have responded to all comments

Reviewer 2 Report

This manuscript demonstrates an extensive review of various types of coating on metal bipolar plates for proton exchange membrane fuel cells (PEMFCs). Appropriate coating techniques to enhance corrosion resistance and electrical conductivity are well reviewed. Therefore, the reviewer recommends that the manuscript be accepted for publication in Energies.

Reviewer 3 Report

The authors has greatly improved the quality of the manuscript. I have no further comment, and believe that the manuscript is ready for publication.