PrBaCo₂O_6−δ-Ce_0.8Sm_0.2O_1.9 Composite Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells: Stability and Cation Interdiffusion

Round 1

Reviewer 1 Report

This paper is well written and provides a detailed analysis of the synthesis and characterisation procedure. Good techniques are used. The information of the manuscript is interesting however, many similar materials are previously studied.

However, prior to publication, the authors may to consider the following remarks:

P1, L14  “as a result”: not clearly understood by me.

 L64 should the author be clarify the statement and also the English there after is unusual.

L77 “all purity is 99.99%: Really? Can you confirm that. it is usually quite uncommon to use all the ingredients in such high purity level.

L84:xrd,  icp and other result in more detail. or any graph of table can be added as extra material.

L209: Cannot agree with the comment and the supporting reference

Fig 7, inset graph should be more clarified

In general, the references from some other group of researcher can add more values to the article.

Author Response

Please note that in the first manuscript draft the second affiliations of some co-authors were omitted by mistake. These affiliations have now been added and highlighted in the text. Please also note that section Funding has been modified by adding essential data. Also please pay attention that all modified and corrected places in the manuscript are highlighted in green except abbreviations PBC and SDC denoting PrBaCo2O6-d and Ce_0.8Sm_0.2O_1.9 electrolyte, respectively, which are highlighted in yellow.

Reviewer 1

Comments and Suggestions for Authors

This paper is well written and provides a detailed analysis of the synthesis and characterisation procedure. Good techniques are used. The information of the manuscript is interesting however, many similar materials are previously studied.

However, prior to publication, the authors may to consider the following remarks:

 P1, L14  “as a result”: not clearly understood by me.

“As a result” serves as a transition phrase here – in fact, it is one of the standard transition phrases linking the ideas of the previous sentence with those of the current one. The previous sentence is “The chemical compatibility, cation interdiffusion, thermal expansion and dc conductivity were studied.” Strong interdiffusion of Pr and Sm was found literally as a result of these studies; hence, we believe that we use “as a result” here appropriately.

 L64 should the author be clarify the statement and also the English there after is unusual.

“However, firstly, such judgment depends very much on annealing conditions, especially on its duration, because of the kinetic limitations of solid state interaction.” This one means that if no new phase was detected after annealing at a certain temperature for a certain time, it doesn’t automatically mean that the materials in question are compatible and do not react with each other. Maybe they do react, but because of the slow solid state reaction rates only a small amount of a new phase is formed. These small amounts of impurities oftentimes cannot be reliably detected with the conventional XRD analysis. This brings us to the next sentence, which we have clarified on the Reviewer’s request in the corrected manuscript: “Secondly, oftentimes, no clear evidence of new phase formation or cation interdiffusion can be assessed on the basis of XRD analysis only, owing to the nonzero detection limits of conventional XRD [22].” We hope that the meaning we intended to convey is now more evident than it was in the previous draft of the manuscript submitted.

L77 “all purity is 99.99%: Really? Can you confirm that. it is usually quite uncommon to use all the ingredients in such high purity level.

We understand this concern of reviewer – pointed out purity may look as too high. We have checked a purity of all the starting materials and can confirm that it matches 99.99% for all the started materials except that for cerium nitrate hexahydrate. The purity of the latter was found to be only 99.9%. For this reason the manuscript has been corrected accordingly. As far as a purity of the starting materials goes it is not uncommon to use 99.99%-pure reagents: for example, Lanhit (http://www.lanhit.ru/) is our primary supplier of 99.99%-pure rare-earth metal oxides and alkaline-earth metal carbonates.

L84:xrd,  icp and other result in more detail. or any graph of table can be added as extra material.

Regarding the ICP results, we believe that no additional information can be added to the manuscript by including those explicitly. The most important result of the chemical composition analysis is that, within the error margins of the method, the materials synthesized are pure and stoichiometric with respect to metals. This conclusion is already pointed out in the manuscript. We feel that no new scientific value could arise from presenting the actual ICP results. The same is true for additional XRD patterns. It is already seen in Figure 3 that XRD patterns of the mixture of PrBaCo₂O_6–δ and Ce_0.8Sm_0.2O_1.9 do not contain any additional impurity peaks, which means that, according to XRD, both PrBaCo₂O_6–δ and Ce_0.8Sm_0.2O_1.9 were initially phase-pure.

L209: Cannot agree with the comment and the supporting reference

The reference represents the particular autobiographical thoughts of Albert Einstein. We hope that the comment and the reference serve only to emphasize the importance of thermodynamic studies. Anyway taking into account the concern of the reviewer we have partly modified this place as “Thereby importance and predictive power of thermodynamics, noticed by Albert Einstein [41], still are actual in the field of materials science.”

Fig 7, inset graph should be more clarified

We hope that the inset in Figure 7 is sufficiently well explained in both the figure caption and the main text (see L257 and onwards). Unfortunately, we do not see which particularities related to the inset could be modified and how they should be modified to add more clarity to already existing explanations. In the absence of any specific requests from the Reviewer, we intend to leave both the inset and the explanations “as is.”

In general, the references from some other group of researcher can add more values to the article.

We agree with Reviewer. Additional references supporting our conclusions have been added (see Refs. [48]-[50] of the corrected manuscript).

Author Response File: Author Response.docx

Reviewer 2 Report

Comments on the article titled “PrBaCo₂O_6-δ - Ce_0.8Sm_0.2O_1.9 composite cathodes for intermediate temperature solid oxide fuel cells: stability and cation interdiffusion”

Comments:

The authors clearly explored the stability and chemical incompatibility of the double perovskite material and ceria cathode composite material base on thermodynamic assessment. Strong interdiffusion of Pr and Sm from PBC and SDC does not improve TEC behavior for cathode and formed poorly conducting material BaCeO3. Authors gently conclude that double perovskite PBC is more suitable as cathode for proton conducting fuel cell using of barium cerate- and barium zirconate-based electrolyte.

However, the authors have to clarify a few queries in the article, which are given below.

The ceramic pellets of PBC and SDC have relatively low density ~90% after sintering at 1250C and 1500C. The sample for conductivity measurement was sintered even low temperature of 1200C and 1300C. It means conductivity sample is not dense.

Figure 1 explains the CO2 effect on chemical stability of PBC but there is no relative experiment in article. If you have this data, it helps that more clear understanding the chemical stability of PBC in operating condition at cathode with SDC.

EDS data shows that BaCeO3 (with Pr dopant; author mentioned 10-20% of Pr) is formed at the interface between PBC and SDC as clear evidence of chemical interaction but this phase is not a detect in XRD. If this peak overlaps with others it needs to mentioned in the article.

According to line 200, it seems that Pr in PBC might diffuse into BaCeO3 or BaZrO3, which is more suitable electrolyte for PBC as mentioned by the author.

Some of the errors to be corrected are

Full name of material and abbreviation are alternately used.

After addressing minor revisions on the above mentioned queries, with valid explanations and correcting the errors, the article may be accepted for the publication in the Energies.

Author Response

Please note that in the first manuscript draft the second affiliations of some co-authors were omitted by mistake. These affiliations have now been added and highlighted in the text. Please also note that section Funding has been modified by adding essential data. Also please pay attention that all modified and corrected places in the manuscript are highlighted in green except abbreviations PBC and SDC denoting PrBaCo2O6-d and Ce_0.8Sm_0.2O_1.9 electrolyte, respectively, which are highlighted in yellow.

Comments:

The authors clearly explored the stability and chemical incompatibility of the double perovskite material and ceria cathode composite material base on thermodynamic assessment. Strong interdiffusion of Pr and Sm from PBC and SDC does not improve TEC behavior for cathode and formed poorly conducting material BaCeO3. Authors gently conclude that double perovskite PBC is more suitable as cathode for proton conducting fuel cell using of barium cerate- and barium zirconate-based electrolyte.

However, the authors have to clarify a few queries in the article, which are given below.

The ceramic pellets of PBC and SDC have relatively low density ~90% after sintering at 1250C and 1500C. The sample for conductivity measurement was sintered even low temperature of 1200C and 1300C. It means conductivity sample is not dense.

It is, indeed, the case. However, the samples used in the present work for the conductivity measurements are of approximately the same density, which means that the conclusions drawn from comparing the conductivities of the samples with different compositions are valid regardless of the particular density value.

Figure 1 explains the CO2 effect on chemical stability of PBC but there is no relative experiment in article. If you have this data, it helps that more clear understanding the chemical stability of PBC in operating condition at cathode with SDC.

The thermodynamic assessment itself was based on the literature data, which was properly referenced in the manuscript [23, 26-28, 30]. As the experimental investigation of interaction of PBC with CO2 was not among the aims of our work, we rely on the data of others [10,22,33-35] to confirm the diagram in Figure 1.

EDS data shows that BaCeO3 (with Pr dopant; author mentioned 10-20% of Pr) is formed at the interface between PBC and SDC as clear evidence of chemical interaction but this phase is not a detect in XRD. If this peak overlaps with others it needs to mentioned in the article.

It was mentioned in the manuscript that all the diffraction peaks can be indexed based on a physical mixture of PBC and SDC, and no third phase is detected. However, we agree with the Reviewer that in case of Pr-doped BaCeO3, its highest-intensity peak overlaps that of SDC. Because of this, and also because of the small amount of the Pr-doped BaCeO3, the chemical interaction cannot be detected by XRD. This information has been added to the manuscript, which has been modified accordingly.

According to line 200, it seems that Pr in PBC might diffuse into BaCeO3 or BaZrO3, which is more suitable electrolyte for PBC as mentioned by the author.

We agree with Reviewer. As it is pointed out in the manuscript (see the first paragraph of Section 3.2), large Pr deficiency in PBC is quite unfavorable. It makes the «one-way» diffusion of Pr highly unlikely. In other words, the interdiffusion has to occur, i.e. the «outgoing» diffusion of Pr must be offset by the «incoming» diffusion of another rare-earth ion. Such ion is Sm in case of PBC – SDC interface. In the case of doped cerates or zirconates mentioned by Reviewer the most typical dopants are rare-earth elements such as Sm, Gd etc. Therefore, interdiffusion of Pr and these dopant cations between PBC and BaCeO₃ or BaZrO₃-based electrolyte seems to be quite likely. In fact, some signs of such interdiffusion were observed in Ref. [49] (please see the corrected manuscript). Moreover, in the same Ref. [49] it was concluded that there was no new phase formation on the interface between BaCeO₃-based electrolyte and PBC. This confirms our conclusion on the stability of double perovskite phase in contact with BaCeO₃ or BaZrO₃-based electrolyte. Of course, Pr diffusion into the electrolyte would suppress its performance due to increasing electronic conductivity. Therefore, it would be better to use double perovskites containing other rare - earth elements instead of Pr. Please, also note that this clarification has been introduced in the corrected version of the manuscript (see text between L270 and L283 in the corrected manuscript).

Some of the errors to be corrected are

Full name of material and abbreviation are alternately used.

We have made the necessary corrections; now the abbreviations are used consistently throughout the manuscript. The places where formulae have been replaced by abbreviations are highlighted in yellow.

Author Response File: Author Response.docx