Undoped Sr₂MMoO₆ Double Perovskite Molybdates (M = Ni, Mg, Fe) as Promising Anode Materials for Solid Oxide Fuel Cells

Round 1

Reviewer 1 Report

The paper titled “Undoped Sr2MMoO6 double perovskite molybdates (M = Ni, Mg, Fe) as promising anode materials for solid oxide fuel cells” is a review summarizing the properties of molybdates in a view of the application in SOFC technologies. I believe this review can be interesting for a broad community and should be published after the minor changes listed below.

• Title – I believe the word “undoped” is redundant
• Abstract- half of it does not apply directly to the main topic of the review.
• The part concerning SOFC operation is too long and redundant, authors should focus more on the particular topic –anodes. This part should be revised and shortened.
• It is not clear to what authors refer to in Figure 2/3/4 (atom positions presumably)
• 6 ln. 191 rhombic should be changed to orthorhombic
• 6 ln. 199 what authors mean by “Shannon system”?
• 6 ln. 200 what the author means by saying that for a tolerance factor greater than 1.05 the tetragonal structure is the most stable? What is the reason for this stability? Generally, the Goldsmith tolerance factor reflects the stability of the perovskite structure and for values outside the cubic window the stability decreases.
• 7 The information about the influence of molybdenum and other transition metals presence within the compound structure should be explained in more detail concerning literature data presented in Table 1. There is no critical analysis of data presented in this table.
• 8 It is not clear why the information about synthesis routes is placed in the structure section of the paper. It is not clear what was the relationship between the synthesis procedure and properties of Sr2NiMoO6–δ. This section is rather chaotic and hard to follow. The information in which synthesis procedure proved to end in single-phase perovskites.
• 9 Table 2 presents the data on TEC from particular sources but there is no information about the reliability of the different values.
• 11 ln. 324-325 The formatting of the references is off.
• 11 ln. 337-339 The information about the stability of the compound is very brief, and it’s not clear what the authors mean by “strongly reduced conditions”.
• 11 ln. 340 Author writes that the Sr2MgMoO6–δ “can have” a particular structure. Does it mean that the material is a mixed-phase? Or does it crystallize in different structures depending on the conditions?
• 16 Table 6 the formatting of all tables should be unified, in the case of Air, 800°C values of conductivity the values from different sources are grouped and for other e.g. H2 800°C is repeated.
• 18 Table 8 is not in the scope of the submitted review.

Author Response

Reviewer 1:

This is a good paper, with worthy findings. Please see my comments in the attached PDF. The paper titled “Undoped Sr2MMoO6 double perovskite molybdates (M = Ni, Mg, Fe) as promising anode materials for solid oxide fuel cells” is a review summarizing the properties of molybdates in a view of the application in SOFC technologies. I believe this review can be interesting for a broad community and should be published after the minor changes listed below.

We are grateful to the Reviewer for his extensive and valuable comments, which have allowed us to improve the overall quality of our work. We have addressed all the comments in the revised manuscript as discussed below. The corresponding changes in the revised manuscript are highlighted in the tracker style. Our response to each point raised by the Reviewers is given below in blue font alongside the relevant comment.

 

Title – I believe the word “undoped” is redundant

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18 Table 8 is not in the scope of the submitted review.

Response: We respectfully do not agree with these two comments. The main objects of the current review article are basic compounds (Sr₂NiMoO_6–δ, Sr₂MgMoO_6–δ, Sr₂FeMoO_6–δ and Sr₂Fe_1.5Mo_0.5O_6–δ) and their physico-chemical and functional properties. Almost all tables and figures are devoted to this goal. In conclusion of our work, we highlight this fact, but give the readers more details about how the basic compounds can be doped and where the corresponding information can be found.

 

Abstract- half of it does not apply directly to the main topic of the review.

Response:  The abstract has been shortened by one phrase. However, it has been prepared according the Materials journal’ guide. In detail, the abstract contains information about background, gap, objects, results, and conclusions.

 

The part concerning SOFC operation is too long and redundant, authors should focus more on the particular topic –anodes. This part should be revised and shortened.

Response:  We agree with this suggestion. However, we should mention that this section contained information about both SOFCs and anodes. In the revised version, the Section 2 has been properly divided into Section 2 (Brief descriptions of SOFCs) and Section 3 (Anode materials for SOFCs)

 

It is not clear to what authors refer to in Figure 2/3/4 (atom positions presumably)

Response:  These figures correspond to molybdates having different distortions of their structure (cubic in Fig. 2, cubic-to-tetragonal transition in Fig. 3 and monoclinic in Fig. 4). The “cubic” has been added in the caption of Fig. 2.

 

6 ln. 191 rhombic should be changed to orthorhombic

Response:  The corresponding change has been made.

 

6 ln. 199 what authors mean by “Shannon system”?

Response: This phrase has been revised as follows:

“where rA, rB and rO are the effective ionic radii of A-, B- and O-ions according to Shannon”

 

6 ln. 200 what the author means by saying that for a tolerance factor greater than 1.05 the tetragonal structure is the most stable? What is the reason for this stability? Generally, the Goldsmith tolerance factor reflects the stability of the perovskite structure and for values outside the cubic window the stability decreases.

Response:  We are noting that our original text said about the stability of the hexagonal structure at t>1.05. Not tetragonal. Yet we are agree, the used term “the most stable” structure was not entirely correct. For clarity we have rewritten our text about the relationship between the tolerance factor t, which is a quantitative evaluation of the perovskite structures instability caused by mismatch between the sizes of a cation in A-position and the octahedral voids, and the type of forming crystal structure.

“Excluding rare examples that arise due to the difficulty of determining variable oxidation states of the cations, the A2MMo'O6 compounds are found to be crystalized in various crustal structures [74]: hexagonal (space groups P6 /mmc, P6 c) at t > 1.05, cubic (Fm m) at 1.00 < t < 1.05, tetragonal at 0.97 < t < 1.00 and, finally, triclinic (P ), monoclinic (P21/n) or orthorombic (Pmm2) at t < 0.97.”

 

7 The information about the influence of molybdenum and other transition metals presence within the compound structure should be explained in more detail concerning literature data presented in Table 1. There is no critical analysis of data presented in this table.

Response:  Table 1 was presented as an example that shows the reached power densities (P) of SOFCs. The critical analysis of this table is not rational because numerous factors affect the P values; as the readers can see, the SOFCs presented in this table were prepared not only from different molybdtates, but different oxygen electrodes as well. Therefore, this table acts as evidence that molybdates are promising anodes.

 

8 It is not clear why the information about synthesis routes is placed in the structure section of the paper. It is not clear what was the relationship between the synthesis procedure and properties of Sr2NiMoO6–δ. This section is rather chaotic and hard to follow. The information in which synthesis procedure proved to end in single-phase perovskites.

Response:  The study of the functional properties of a number of complex oxides with a double perovskite structure (Sr2MMoO6, in particular: Sr2NiMoO6 [81], Sr2MgMoO6-δ [108], Sr2Ni0.7Mg0.3MoO6-δ [143], Sr2Ni0.75Mg0.25MoO6) has showm that the molybdate synthesis pathway plays a critical role in the properties of target materials. For example, the authors of [81] specially note that the synthesis technique (solid state synthesis or solution methods) significantly affects the type of structure and electrical properties of the obtained Sr2NiMoO6 samples. The authors of [108] showed that the microstructure and electrical conductivity of Sr2MgMoO6-δ samples substantially depend on the calcination conditions (temperature and type of atmosphere). In [143], it was concluded that even the type and relative amount of the organic component during the pyrolysis synthesis of the Sr2Ni0.7Mg0.3MoO6 had an impact on the phase stability, thermal expansion and electroconductivity of the final material. We agree with the reviewer that in Section 4.1, the idea of ​​a direct relationship between the method for the synthesis of Sr2NiMoO6 and the target properties of the obtained oxide material was not raised. Therefore, the following text has been added:

“As shown in works [25, 81], the synthesis methods of the Sr₂MMoO_6–δ phases determine their phase compositions, crystal structures, microstructural morphologies, and physico-chemical properties; in particular, electrical transport properties and thermodynamic stability are considerably varied depending on pre-history of these materials. This comes from the fact that the preparation pathways and the synthesis/sintering conditions of Sr₂MMoO_6–δ determine the content of Mo⁺⁵ ions (equation 18) in the resulting phases  [75]. As a result, a high content of Mo⁺⁵ ions in Sr₂MMoO_6–δ governs stability of the final oxides in reducing atmospheres [25] and high values ​​of electrical conductivity [75]. Therefore, when characterizing the structure and properties of Sr₂MMoO_6–δ, their preparation details should be thoroughly analysed.”

 

9 Table 2 presents the data on TEC from particular sources but there is no information about the reliability of the different values.

Response: Usually, the TEC data are calculated from linear parts of dilatometric curves. Depending on their bends, local deviations, some errors in the TEC determination can appear. However, these errors are quite low (below 5% from the calculated TEC). Moreover, the cited works do not provide the reliability information. Due to this fact, we reuse the presented TEC from these works.

 

11 ln. 324-325 The formatting of the references is off.

Response:  All references have been upgraded according to the Materials journal requirements.

 

11 ln. 337-339 The information about the stability of the compound is very brief, and it’s not clear what the authors mean by “strongly reduced conditions”.

Response:  This phrase has been revised as follows:

“…Sr₂MgMoO_6–δ does not decompose (in contrast to Sr₂NiMoO_6–δ) in H₂-containing atmospheres at high temperatures”.

 

11 ln. 340 Author writes that the Sr2MgMoO6–δ “can have” a particular structure. Does it mean that the material is a mixed-phase? Or does it crystallize in different structures depending on the conditions?

Response:  Thank you for this remark. The correct is the second variant. The corresponding phrase has been revised.

 

16 Table 6 the formatting of all tables should be unified, in the case of Air, 800°C values of conductivity the values from different sources are grouped and for other e.g. H2 800°C is repeated.

Response:  Thank you for this suggestion. Considering all tables presented in this review, we decide to format Tables according to the reference criterion.

Reviewer 2 Report

In this review article, the authors reviewed recent works on Sr2MMoO6 double perovskites as SOFC anode candidate materials. The article is well organized and informative. It thoroughly includes basic quantities, such as crystal structure, thermal expansion coefficients, as well as the chemical stability of these materials. I recommend acceptance with minor revision.

1. Defect chemistry of these materials is lightly touched in the second section. But it may worth a closer view. As listed by the author, the M site cation plays a great role in altering the electronic structures across these compositions. Such a change in electronic structure is expected to result in different defect chemistry. Please consider reviewing the similarity and difference of defect chemistry of these double perovskite materials.

2. A site defects and oxygen vacancies in perovskites are known to induce phase decomposition near surfaces/interfaces and lead to performance degradation. Is this the case for Sr2MMoO6? How do synthesis methods and thermal history affect material stoichiometry? And are there any known atomic mechanisms that link stoichiometry with chemical stability?

3. Most of the reviewed work focus on the intermediate-temperature range. The authors should include why these materials are not suitable for low- and high-temperature SOFCs.

4. Comparison with the state of art anode candidates (that are not Sr2MMo6 type) are not sufficient. Please consider adding more comparisons in terms of cost, power density, and chemical stability.

Author Response

We are grateful to the Reviewer for his extensive and valuable comments, which have allowed us to improve the overall quality of our work. We have addressed all the comments in the revised manuscript as discussed below. The corresponding changes in the revised manuscript are highlighted in the tracker style. Our response to each point raised by the Reviewers is given below in blue font alongside the relevant comment.

 

1. Defect chemistry of these materials is lightly touched in the second section. But it may worth a closer view. As listed by the author, the M site cation plays a great role in altering the electronic structures across these compositions. Such a change in electronic structure is expected to result in different defect chemistry. Please consider reviewing the similarity and difference of defect chemistry of these double perovskite materials.

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2. A site defects and oxygen vacancies in perovskites are known to induce phase decomposition near surfaces/interfaces and lead to performance degradation. Is this the case for Sr2MMoO6? How do synthesis methods and thermal history affect material stoichiometry? And are there any known atomic mechanisms that link stoichiometry with chemical stability?

Response: The discussions corresponding to the chemical stability of the reviewed phases and relations with oxygen content have been added for each system (sections 5.1, 5.2, 5.3):

“The mentioned phase relation for Sr₂NiMoO_6–δ in reducing atmospheres might be explained by a low redox ability of Ni-ions. The latter are exsolved from the perovskite structure until the formation of metallic Ni-particles and unstable “Sr₂MoO₆” residue that decomposes into a number of more simple molybdates.”

“More precisely, the impurity phases of SrMoO₄ and Sr₃MoO₆ are formed along with the target Sr₂MgMoO_6–δ compound after its synthesis in an air atmosphere [98]. Although Mg-ions exhibit very high chemical stability due to the constant oxidation state (+2), they cannot compensate an excess of the lattice oxygen upon the oxidation of Sr₂MgMoO_6–δ, leading to phase decomposition.”

“Chemical stability of Sr₂FeMoO_6–δ as a representative of a Sr₂Fe_1–xMo_xO_6–δ family [116, 118, 119] is caused by the existence of two redox active elements showing a variety in their oxidations states (+2, +3 and +4 for iron and +5 and +6 for molybdenum). In reducing atmospheres, these cations exist in reduced states (Fe⁺², Fe⁺³, Mo⁺⁵, Mo⁺⁶) adjusting the oxygen content below 6.0 (i.e. δ > 0). In atmospheres with high oxygen partial pressures, the content of oxidized cations (Fe³⁺, Fe⁴⁺, Mo⁶⁺) increase, leading to unstable over-stoichiometry products decomposed until the formation of a SrMoO₄ impurity.”

According to our opinion, the description of defect chemistry is redundant for the current review. Instead of defect chemistry, some information about oxidation states has been provided.

3. Most of the reviewed work focus on the intermediate-temperature range. The authors should include why these materials are not suitable for low- and high-temperature SOFCs.

Response: The following parts have been added to address this comment:

“According to [82, 91, 96], the double perovskite molybdates interact with YSZ electrolytes with the formation of SrMoO₄ (at temperatures above 800 °C) and SrZrO₃ (at temperatures above 1000 °C) phases. For this reason, Sr₂MMoO₆ cannot be considered as anode materials for high-temperature SOFCs.”

 “One of the requirements for anode materials [50] is their high electronic conductivity for effective electrical connectivity with interconnectors. From Tables 3, 5 and Figure 13a, it can be concluded that the electrical conductivity of the Sr₂MMoO₆ (M = Ni, Mg, Fe) molybdates below the required values (10 S cm^–1) in reducing atmospheres at low temperatures; therefore, they can be used for intermediate-temperature SOFCs.”

4. Comparison with the state of art anode candidates (that are not Sr₂MMo₆ type) are not sufficient. Please consider adding more comparisons in terms of cost, power density, and chemical stability.

Response: Table 1 has been extended by new data, whie the corresponding discussion has been added:

In Table 1 the power density values for the traditional Ni–YSZ cermet anodes were also presented. From this comparison one can seen that molybdates Sr₂MMoO_6–δ (M = Ni, Mg, Fe) are not inferior to the traditional anode in terms of power densities. Moreover, in case of the Ni-cermet anodes, the carbon particles formed during hydrocarbon fuel pyrolysis are deposited on the electrode surface that leads to the cell degradation [82]. The problem of coke formation may be solved by incorporating a catalyst (Au, Pd, Ru) into the Ni-based cermet anodes to avoid the conditions of coke-formation. Obviously, it will increase the cost of the Ni-cermet anodes, which does not seem to be economically viable. The investigations of carbon deposition behaviors of the Ni–YSZ-based anodes after treatment in biogas [83] and the Sr₂MnMoO_6–δ/NiO–Ce_0.8Sm_0.2O_1.9 composite in methane [28] illustrate the undoubted advantages of the molybdate-based anodes.

Reviewer 3 Report

In this review article, the authors intend to provide an overview of recent applications of Sr₂MMoO₆ double perovskite molybdates (M = Ni, Mg, Fe) as anode materials for SOFC. In my opinion, the manuscript requires editing before it can be considered for publication. At this stage, I recommend a “minor revision”. My comments are listed below:

• Line 13-15: “This is due to the inhibition of electrochemically active sites, irreversaible changes in the microstructural parameters and thermal and mechanical strains.” Rephrase this statement
• Line 18-21: “The main emphasis is devoted to the synthesis features of undoped double molybdates, their electrical conductivity and thermal behaviors in both oxidizing and reducing atmospheres, as well as their chemical compatibility in respect to other functional SOFC materials and as components of fuel.” Rephrase this statement
• Line 260-261: “In [69], Authors used the lyophilization of an aqueous solution of cations Tto obtain Sr₂NiMoO_6–δ.” Correct the typos
• For better comparison, data presented in tables 2, 4 and 7 should be presented in a single figure using chart/histogram.
• For better comparison, data presented in tables 3, 5 and 6 should be presented in a single figure using chart/histogram.
• Figures 5 and 6 should be merged and renumbered as (a), (b), (c), (d) respectively
• Figures 7 and 9 should be merged and renumbered (a), (b) respectively
• Line 233, 318, 394, 438: renumber the sections as “4.1”, “4.2”, “4.3” and “4.4” respectively

Author Response

Reviewer 2:

In this review article, the authors intend to provide an overview of recent applications of Sr₂MMoO₆ double perovskite molybdates (M = Ni, Mg, Fe) as anode materials for SOFC. In my opinion, the manuscript requires editing before it can be considered for publication. At this stage, I recommend a “minor revision”.

We are grateful to the Reviewer for his extensive and valuable comments, which have allowed us to improve the overall quality of our work. We have addressed all the comments in the revised manuscript as discussed below. The corresponding changes in the revised manuscript are highlighted in the tracker style. Our response to each point raised by the Reviewers is given below in blue font alongside the relevant comment

 

My comments are listed below:

Line 13-15: “This is due to the inhibition of electrochemically active sites, irreversaible changes in the microstructural parameters and thermal and mechanical strains.” Rephrase this statement

Response:  This phrase has been deleted according to comments of the Reviewer 1.

 

Line 18-21: “The main emphasis is devoted to the synthesis features of undoped double molybdates, their electrical conductivity and thermal behaviors in both oxidizing and reducing atmospheres, as well as their chemical compatibility in respect to other functional SOFC materials and as components of fuel.” Rephrase this statement

Response:  This phrase has been clarified by separating the text with (i), (ii) and (iii) markers.

 

Line 260-261: “In [69], Authors used the lyophilization of an aqueous solution of cations Tto obtain Sr₂NiMoO_6–δ.” Correct the typos

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Line 233, 318, 394, 438: renumber the sections as “4.1”, “4.2”, “4.3” and “4.4” respectively

Response:  The corresponding changes have been made.

 

For better comparison, data presented in tables 2, 4 and 7 should be presented in a single figure using chart/histogram.

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For better comparison, data presented in tables 3, 5 and 6 should be presented in a single figure using chart/histogram.

Response: These comments have been addressed in the revised version of the manuscript. Since the suggested figures repeat the table data, we present these Figs in Appendix A (see Figs A1 and A2).

 

Figures 5 and 6 should be merged and renumbered as (a), (b), (c), (d) respectively

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Figures 7 and 9 should be merged and renumbered (a), (b) respectively

Response:  Figures 5 and 6 show different properties: phase relations of the basic (single-phase) Sr₂NiMoO_6–δ compound after its reduction in H₂ and chemical compatibility of the same phase with various solid state electrolytes. Due to this fact we would like to maintain the original data with no changes. The same aspect concerns with Figs 7 and 9.