Fluorine Anion-Doped Ba0.6Sr0.4Co0.7Fe0.2Nb0.1O3-δ as a Promising Cathode for Protonic Ceramic Fuel Cells
Round 1
Reviewer 1 Report
The manuscript «Fluorine anion-doped Ba0.6Sr0.4Co0.7Fe0.2Nb0.1O3-δ as a promising cathode for protonic ceramic fuel cells» presents the new information about properties of F-doped Ba0.6Sr0.4Co0.7Fe0.2Nb0.1O3-δ. The authors use modern methods of investigations and the obtained phases are well certified.
The authors clearly define the research problem, they clearly show the importance of this study and the results would be worthy for publication, but some further clarification is needed a major revision is needed before this article can be considered for publication.
1. The authors suggest the following mechanism for the introduction of fluorine, depending on its concentration: first, fluorine ions occupy oxygen vacancies and interstitial sites, and with increasing concentration, fluorine ions begin to occupy the positions of oxygen ions (р.8). Then, this will lead to a different character of the change in the density of the samples. Therefore, this conclusion must be confirmed by calculations of the X-ray density of the samples. If possible, it would be good to compare with experimental densities.
2. The authors present the results of the EDS mapping and confirm the homogeneous distribution of the elements (p.5). However, why do not they show data on elemental analysis. Moreover, further the authors carry out calculations of oxygen nonstoichiometry based on the oxidation states of the elements. That is, the authors must indicate the elemental composition of the phases, and then, having data on the oxidation states of the elements, carry out calculations of nonstoichiometry. As a rule, it is possible to expect a lack of barium and strontium for such phases.
3. Regarding conductivity data. It is strange that the authors do not use generally accepted coordinates “log conductivity – 1/T” ? The activation energies should be calculated and the data discussed.
4. Nonmonotonic change in conductivity with a change in the concentration of fluorine, the authors explain as “increase in the valence state of the Co ions” and with increasing F-concentration “lower valence state of Co ions”. That is, the authors talk about a change in the concentration of electron carriers. These considerations should be commented by quasi-chemical equations (how does the concentration of holes (?) change with different mechanisms of fluorine introduction).
5. Regarding IR data. The authors say that F-doped samples “have higher -OH adsorption capacity”.
This conclusion is not clear, since Fig.5 shows that the F-doped samples have a δ-parameter of less than the sample not doped with fluorine, that is, the concentration of oxygen vacancies is lower.
As a result, it is not clear whether fluorine doping leads to an increase or decrease in oxygen vacancies. In the Abstract, the authors write “fluorine ion doping increases oxygen vacancies”, but in Fig.5 the opposite trend is shown - Oxygen vacancy concentration decreases (~0,57 -0,45 -0,50).
Authors must agree to all data and statements.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
This work on fluorine doped pervoskite cathode is good. Small percetantage of F doping can increase the performance of cathode and overall cell performance. Structure and English standard of the paper is fine. I recommend this work to be published as it is.
Author Response
Responses to Reviewer 2’s Comments and Suggestions
Comments: This work on fluorine doped pervoskite cathode is good. Small percetantage of F doping can increase the performance of cathode and overall cell performance. Structure and English standard of the paper is fine. I recommend this work to be published as it is.
Response: We appreciate the reviewer’s comments on our manuscript. We are very sincerely grateful to the reviewer for the permission to publish this paper, and thank the reviewer for his/her recognition of our work.
Reviewer 3 Report
Authors present an interesting paper concerning a new compound for cathode for protonic ceramic fuel cells. Nevertheless, some physical characterizations are incomplete. Impedance spectroscopy results are not seriously analyzed, and must be completed.
Page 1, line 43: what does “transparent” mean?
Page 2, line 91: Authors named symmetrical cells as BSCFN / BSCFN-Fx | BZCYYb | BSCFN / BSCFN-Fx. We suggest, in order to avoid any ambiguity, to write : BSCFN (or BSCFN-Fx) | BZCYYb | BSCFN (or BSCFN-Fx).
Page 3, lines 128 to 131: Authors write “Compared to the calculated lattice parameter of BSCFN, the BSCFN-F0.05 and BSCFN-F0.1 have smaller lattices, which is in line with the law that ions with small radii are incorporated into the lattice to cause lattice shrinkage”. Nevertheless, the difference in parameters are not significant… So we suggest authors to be more careful, and just say that these result “are in accordance with a very small lattice shrinkage”…
Page 5, lines 153 to 155: Authors write: “As shown in Figure 3b (local magnification of Figure 3a), the characteristic peak of F is located at 683 eV, indicating that fluorine ion was successfully incorporated into BSCFN”. According to caption, Figure 3b shows results only for the sample BSCFN-F0.1. Please give also results for BSCFN-F0.05 and BSCFN for comparison… Perhaps they correspond to the two other curves of Figure 3b: in this case, for BSCFN-F0.05, the peak intensity is very low, nearly not significant… Why?
Page 7, line 202: What is the “titration method”? Please clarify.
Page 9, lines 254-256: Authors write “Obviously, fluorine ion doping produces a stronger OH absorption band than BSCFN, indicating that both BSCFN-F0.05 and BSCFN-F0.1 have higher OH adsorption capacity, which promotes hydration to a certain extent.” In fact, the differences on Figure 7b are not obvious. It seems necessary to complete Figure 7b with a zoom of the 3300-3600 cm-1 range.
Page 9, lines 259-267: Figures 8a, 8b and 8c are useless and must be suppressed. In order to give sense to a Nyquist diagram, it is necessary to indicate frequencies. At this stage, authors have just measured the low frequency resistance of the diagrams, so all data are in Figure 8d. If authors want to show impedance diagrams, they must give a serious analysis, with fit and capacity determinations…
Page 10, lines 276-277: Authors assert “RE1 and RE2 represent the charge transfer process and the oxygen surface process, respectively”. Normally, it is possible to interpret the origin of the resistance after calculating the corresponding capacitances. Authors must give the values of these capacitances, and not only T and P parameters (Table 4). Authors give as explanation a reference corresponding to a completely different compound.
Page 11, Table 4: The value of Rohm are different for the three samples. Why? You must explain this fact… Moreover, if these values are not the same for the complete cells, it is impossible to say that differences of power values of Figure 10 are only due to electrodes…
Page 11, lines 294-300: The DRT analysis bring no information and must be suppressed… or completely written again. Indeed, authors separate Three zones, LF, MF and HF, whereas there are at least 6 phenomena. Moreover, in Nyquist diagrams, authors have only separated to phenomena, corresponding to RE1 and RE2 values. All these inconsistencies must be corrected.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 4 Report
In this work entitled „Fluorine anion-doped Ba0.6Sr0.4Co0.7Fe0.2Nb0.1O3-δ as a promising cathode for protonic ceramic fuel cells”, Fluorine anion-doped Ba0.6Sr0.4Co0.7Fe0.2Nb0.1O3-x-δFx (x = 0, 0.05, 0.1) perovskites have been synthesized and evaluated as cathode materials for protonic ceramic fuel cells. The crystal structure properties of Ba0.6Sr0.4Co0.7Fe0.2Nb0.1O3-x-δFx (x = 0, 0.05, 0.1), morphology and phase composition of BSCFN-F0.1 using HR-TEM, conductivity of BSCFN-Fx, electrochemical properties (including electrode polarizations and cell performance) of constructed cells and the durability have been investigated in the manuscript. an anode-supported single cell with the configuration of Ni-BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) | BZCYYb | Ba0.6Sr0.4Co0.7Fe0.2Nb0.1O3-0.1-δF0.1 (BSCFN-F0.1) achieved a high peak power density of 630 mW cm-2 at 600 °C. The work is well organized and of great interest for the readers of the journal of Catalysts.
However, the power output presented in Figure 11a is much lower, and it should be clearly clarified. As shown in Figure 11a, the BSCFN-F0.1 cathode shows durability of ~120 h at a current density of 0.16 A cm-2, indicating a much lower power output (around 150 mW cm-2) than 630 mW cm-2 at 600 °C.
The thickness of every layer (electrode layer, electrolyte) in symmetrical cells with BSCFN / BSCFN-Fx | BZCYYb | BSCFN / BSCFN-Fx, PCFCs with BSCFN / BSCFN-Fx | BZCYYb | NiO-BZCYYb should be given. The working area of cathode and the gas flow rate (air, hydrogen) should be presented in the manuscript.
The obtained electrochemical performance results of the cells can be compared with other literature data. A table containing the results and literature data could be helpful.
The authors stated that “The electronic conductivity of the BSCFN and BSCFN-Fx (x=0.05, 0.10) in the range of 300-900 °C was evaluated by the four-probe test method”. How did the authors measure the pure electronic conductivity not the total electrical conductivity. Did the authors collect the data in dry air? In the experimental part, the conductivity measurement details should be included.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The article can be accepted for publication
Author Response
Dear Reviewer,
We are very sincerely grateful to you for the permission to publish this paper, and thanks for your recognition of our work.
Yours sincerely,
Dr. Chao Su
Reviewer 3 Report
Authors have seriously correct this paper, which is now suitable for publication.
Author Response
Dear Reviewer,
We are very sincerely grateful to you for the permission to publish this paper, and thanks for your recognition of our work.
Yours sincerely,
Dr. Chao Su
Reviewer 4 Report
As the conductivity collected by the authours is the total conductivity, the statement of "electronic conductivity" should be change to "electrical conductivity". After the revision, the work can be accepted for publicaiton.
Author Response
Dear Reviewer,
We are grateful to the reviewer for the time and efforts spent on reviewing this manuscript.
Based on the reviewer's suggestion, we have changed "electronic conductivity" to "electrical conductivity" in the revised manuscript.
Thank you so much!
Yours sincerely,
Dr. Chao Su
