Investigation of Electrochemical Characteristics and Interfacial Contact Resistance of TiN-Coated Titanium as Bipolar Plate in Polymer Electrolyte Membrane Fuel Cell
Round 1
Reviewer 1 Report
Reviewer’s Comments
The manuscript is rich in scientific contents and is worth publishing. However, the following suggestions in my opinion will certainly further improve the manuscript:
1. “Investigation on” should be replaced by investigation of in the title. Also, Tin coated Titanium is written in title but in the abstract, it is TiN coated titanium, please check! Typo error, I think.
2. The letters PEMFC should be written in bracket after the full name in the text
3. The letters PVD appeared twice in the Keywords.
4. Figure 5a – line 178 – I think is referring to Figure 5b. please check!
5. In Equation 4 angular frequency is represented by w where in line 213 ω does not match.
6. Numbering Figure 7 and citing it in the text (are not in sequence) requires some attention.
7. Figure 8 is combined with a table. The table should have its own caption and should be referred to in the text.
8. What is IBAD in the table in Figure 8?
9. Overall, the English language of the paper requires some improvement and must be revised.
Author Response
Dear Reviewer:
Please find enclosed our response to the reviewer's comments “Investigation of electrochemical characteristics and interfacial contact resistance of TiN coated titanium as bipolar plate in polymer electrolyte membrane fuel cell,” which we request you to consider for publication as a research article in Coatings.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors are describing the fabrication and materials characterization of TiN coatings on Ti bipolar plates, using various experimental techniques. Most of the results are clearly described and interpreted, however, there are a few questions and unclarities that should be addressed in a revised version of the manuscript. The comments are listed below:
Line 4
Correct typo in the title: [Electroylte]
Line 12, Abstract
[PEMFC] should be written in full at first time use
Same comment for line 30, where [PEMFC] is used for the 1st time in the main text
Line 46
[poor electrical conductivity]
Can the authors quantify this to some extent ? For instance, add a comment: [Metal X / Y / Z has a factor of ..... lower electrical conductivity than graphite.]
Line 61
[Grade 1 titanium]
Is that a generic term ? To me, it is not immediately clear what this comment is referring to. Consider to remove or explain clearly what that actually means.
Line 77
[US/PCI4/750 (Gamry) was used]
I assume that is a potentiostat ? That should be clearly mentioned.
Line 85
[recommended by the DoE]
As this seems to be a recommended standard procedure by the DoE, the authors should include a reference to either a publication or a standard procedure number of the DoE.
Line 86-87
[After 60 min of initial delay,]
Why is that ‘delay’ used ? If that is for stabilization or otherwise, that should be mentioned.
Line 87-88
[potentiodynamic polarization tests were performed at open circuit potentials of −0.25 V to 1.6 V with a scan rate of 1.0 mV/s.]
Please formulate clearly. A potentiodynamic test can not be performed over a range of open-circuit potentials.
Line 94
[immersion in the cathode (+0.6 VSSCE) and anode (−0.1 VSSCE).
‘Immersion IN the cathode’ > Wrong English formulation, it seems. Please reformulate.
Line 101
[the value]
Which value is measured ? That is unclear from the text.
Line 109-110
[EDS analysis showed that Ti = 81.94%, O = 13.17% for the titanium substrate]
Do the authors have an explanation for such a high oxygen concentration in the metallic substrate ?
Furthermore, the authors should be clear about what these percentages exactly are (weight % or atomic % ? I assume the latter).
When reading the figure caption of figure 2 (line 121), it becomes clear the percentages are mass %.
Line 123
[CrN coated ]
This seems to be incorrect.
Line 126
The authors should somewhere in the manuscript explain what the reason is that fluoride is present in the solution. Is that for keeping the surface oxide free during anodic polarization, or otherwise ?
In the discussion of the Tafel plots in Figure 4, the authors should explain what the exact procedure is by which they extract the corrosion current density. Which potential range around the corrosion potential is used for drawing the extrapolation lines ?
Line 137-156
Question regarding porosity determination from the electrochemical measurements: how accurate is this method ? What are typical variations in the calculated value, when the experiment is repeated multiple times ?
Have the authors cross-checked the calculated porosity with other methods, that do not rely on an electrochemical measurement ?
How should ‘porosity’ be interpreted in this context ? This is different than the pinholes that are shown later in figure 6, or are these related ?
Line 163-165
[The conventional potentiostatic corrosion experiment for evaluating the durability of a PEMFC bipolar plate has different and conflicting results due to the different concentrations of sulfuric acid and hydrofluoric acid [23–25].]
This description is somewhat vague and not concrete and sufficiently clear. How big are the differences between the presently discussed results and the values using a more conventional method ? What does conflicting mean in this context ? Completely opposite trends, or orders of magnitude difference, ..... ? All that is not clear from the current way of formulating, therefore, please clarify. What are the sulfuric acid and hydrofluoric acid concentrations used in the conventional method ?
Line 178
[Figure 5(a)] > [Figure 5(b)]
Line 189
Equation 3 and the interpretation and discussion of it on line 184-188.
What is the role and impact of fluoride ions present in the solution on the dissolution of titanium oxide on the surface ? How do the current transients look like in a solution in which there is no fluoride present ?
Line 190-194
What is the number of pinholes per unit surface area ? Have the authors determined that value ?
Figure 8, table
What is [IBAD] ?
I think that is nowhere explained.
Line 263
[current corrosion density] > [corrosion current density]
Conclusions section: personally, I do not like the listing of conclusions and having them labelled individually, as it reads a bit awkward. Please consider to adapt it into a more easily readable section containing one or more paragraphs.
Author Response
Dear Reviewer:
Please find enclosed our response to the reviewer's comments “Investigation of electrochemical characteristics and interfacial contact resistance of TiN coated titanium as bipolar plate in polymer electrolyte membrane fuel cell,” which we request you to consider for publication as a research article in Coatings.
Author Response File: Author Response.pdf
Reviewer 3 Report
The paper deals with electrochemical characteristics and interfacial contact resistance of tin coated titanium electrodes for fuel cells. The experimental part is adequately described and the obtained results are sound. The conclusions are clearly formulated in order to be understood by practitioners and permit the further deployment of PEMFCs.
Author Response
Dear Reviewer:
Please find enclosed our response to the reviewer's comments “Investigation of electrochemical characteristics and interfacial contact resistance of TiN coated titanium as bipolar plate in polymer electrolyte membrane fuel cell,” which we request you to consider for publication as a research article in Coatings.
Author Response File: Author Response.pdf
