Recent Advances of First d-Block Metal-Based Perovskite Oxide Electrocatalysts for Alkaline Water Splitting

Round 1

Reviewer 1 Report

The present review emphasis on the First d-block metal-based perovskite oxides (FDMPOs)for electrocatalytic water splitting. The review discusses the various type of perovskite oxides, their general synthesis, and strategies such as the creation of vacancies, defect engineering, epitaxial mismatching, etc to improve the water electrolysis performance for HER, OER and overall water splitting. The review is focused on an important aspect of alternative fuel generation from water using earth-abundant materials. The review is well written and concise. I recommend publication after a few changes.

1. To attract a wide audience the introduction should be modified by explaining the importance of water splitting, alternative energy fuels, etc.
2. How efficient are FDMPOs in comparison to other catalysts such as doped carbon materials and inorganic oxides? Some numbers should be provided which shows the efficiency of electrocatalytic water efficiency.
3. The Figures resolution is very poor, and It should be fixed. It is mandatory. The author should download high-resolution figures or can request authors for the same.
4. There are few grammatical, sentence, and phrasal errors.
5. The authors have underlined the manuscript in someplace. Is this because of editing in track change mode? The essential requirement is the author should check similarity content with other publications and review to avoid any ethical conflict.
6. Although the review is focused on water splitting in alkaline conditions, however, I can see any electrocatalytic performance related discussion and figures. 

Author Response

We thank the reviewers for their careful and insightful comments. We are delighted to receive so many useful suggestions, which have greatly improved the quality of our work. We think that both the readability and the scientific content are greatly improved after incorporating the reviewers’ advice into the manuscript. In the following response, we will answer the reviewers’ questions to the best of our abilities. Please find our responses below. All revisions are highlighted in the manuscript.

 

Reviewer #1: - The present review emphasis on the First d-block metal-based perovskite oxides (FDMPOs)for electrocatalytic water splitting. The review discusses the various type of perovskite oxides, their general synthesis, and strategies such as the creation of vacancies, defect engineering, epitaxial mismatching, etc to improve the water electrolysis performance for HER, OER and overall water splitting. The review is focused on an important aspect of alternative fuel generation from water using earth-abundant materials. The review is well written and concise. I recommend publication after a few changes.

1. To attract a wide audience the introduction should be modified by explaining the importance of water splitting, alternative energy fuels, etc.

Response: Many thanks to the reviewer for recognizing our work. We agree with the reviewer’s suggestion and have added some background information regarding water splitting and alternative energy fuels in the Introduction. This added part is also attached here for your reference: “Water electrolysis is gaining extensive interest for clean production of hydrogen, with the advantages of high efficiency, abundant sources, little environmental impact, etc. In addition, by storing the electricity in the form of chemical energy (i.e. hydrogen), water electrolysis helps mitigate the intermittent issue of renewable energies (e.g. wind, solar).”Thanks again for this comment.

 

Reviewer #1:

2. How efficient are FDMPOs in comparison to other catalysts such as doped carbon materials and inorganic oxides? Some numbers should be provided which shows the efficiency of electrocatalytic water efficiency.

Response: Thank the reviewer for this comment. Since various categories of materials have been studied for alkaline water electrolysis, a direct comparison of their performance is not that straightforward, in particular the testing setup and conditions vary among different studies. For example, for FDMPOs, they usually have limited surface area, restricting their active site densities. In contrast, carbon-based materials are usually characterized by large surface areas. As a result, it is not that fair to compare their geometric activity directly. In most cases, benchmark Ru-/Ir-based catalysts for OER and Pt-based catalysts for HER are commonly used for comparison. In this review, we also compared the water electrolysis performance with those benchmark catalysts. Also, in the “Perspectives and challenges” part, we point out “Although many perovskite compositions have been reported to outperform Ru- or Ir-based electrocatalysts in catalyzing the OER, the alkaline HER activity of many perovskite oxides is still far below that of Pt metal/alloys”. Thanks again for this comment.

 

Reviewer #1:

3. The Figures resolution is very poor, and It should be fixed. It is mandatory. The author should download high-resolution figures or can request authors for the same.

Response: Thank the reviewer for this comment. We are sorry for the poor resolution of the figures in the main text. We have replaced them with the high-resolution figures. Thanks again for this comment.

 

Reviewer #1:

4.There are few grammatical, sentence, and phrasal errors.

Response: Thank the reviewer for this comment. Our manuscript has been pre-checked by Editage (www.editage.co.kr) for English language editing. Following the reviewer’s comment, we double-checked the writing again to minimize the writing issues. Thanks again for this comment.  

 

Reviewer #1:

5. The authors have underlined the manuscript in someplace. Is this because of editing in track change mode? The essential requirement is the author should check similarity content with other publications and review to avoid any ethical conflict.

Response: Thank the reviewer for this comment. We are sorry for this format problem, which likely arises during the uploading process. We have modified the format according to the requirement of Catalysts. Moreover, we paraphrase previous works with our own words to minimize the similarity issues. Thanks again for this comment.  

 

Reviewer #1:

6. Although the review is focused on water splitting in alkaline conditions, however, I can see any electrocatalytic performance related discussion and figures.

Response: Thank the reviewer for this comment. As introduced in the manuscript, FDMPOs are attempted for the electrocatalysis of the hydrogen evolution reaction (HER) and the overall water splitting reaction recently. The number of papers applying FDMPOs into HER and overall water splitting is small compared with the corresponding OER study. However, this review covers most of the current representative studies applying FDMPOs for HER and overall water splitting. Please find the descriptions from Figure 1b, Figure 4a, and Figure 5b for HER, and Figure 3a, Figure 3c, Figure 4b, Figure 8b for overall water splitting. We also summarized their performance in Table 1. In the Challenges and Perspectives part, we point out that  “the alkaline HER activity of many perovskite oxides is still far below that of Pt metal/alloys”, “In addition to the half-cell OER/HER test, it is very crucial to build the practical water electrolyzer and exam the realistic performance where other practical and yet-unsolved issues such as the durability and catalyst loading can be clearly identified”. Thanks again for this comment.

Author Response File: Author Response.pdf

Reviewer 2 Report

 

This is a comprehensive review on the progress of perovskite oxide catalysts for electrolysis of water. Insight analyses have been carried out. It can be accepted for publication after addressing the following minor issues.

1. The quality of all plots need to be improved. The resolution is too low. I am not sure this happened when the Word file is converted into PDF or, the resolution of the original plots is not high enough.

2. Some plots, the left side, margin too narrow. Only half of the letters 'a1, b1...' are displayed (for example, Figures 6 & 7).

3. What is the source of the data/plot in Figure 2 ? Please cite it.

4. Some text have larger font size with underlines, this is not allowed in journal papers. Please revise this.

5. lines 130-132,'Since the highly oxidizing environments during the OER may oxidize the lattice oxygen,[46] the stability of the oxygen vacancies is also worth studying.' 

I think the highly oxidising environment will affect the stability of the oxide or oxide lattice, not the oxygen vacancies.

6. lines 175-176, 'In addition, solid thin films are not suitable for practical application due to the limited number of active sites available.' This statement is incorrect.

7. lines 221-223, 'We should note that supports or substrates are usually required for exsolution-prepared nanoparticles or deposition-derived nanofilms, which potentially limits the number of active sites available.' This is not correct. Exsolution can happen on nanoparticles without substrates.

8. some language errors, for example,

lines 310-311, 'has been also used' should be 'has also been used'

'simple perovskites' should be 'primitive perovskites'

There are other language errors too.

 

 

 

 

 

 

 

 

 

 

 

 

 

Author Response

We thank the reviewers for their careful and insightful comments. We are delighted to receive so many useful suggestions, which have greatly improved the quality of our work. We think that both the readability and the scientific content are greatly improved after incorporating the reviewers’ advice into the manuscript. In the following response, we will answer the reviewers’ questions to the best of our abilities. Please find our responses below. All revisions are highlighted in the manuscript.

 

Reviewer #2: -

This is a comprehensive review on the progress of perovskite oxide catalysts for electrolysis of water. Insight analyses have been carried out. It can be accepted for publication after addressing the following minor issues.

 

1. The quality of all plots need to be improved. The resolution is too low. I am not sure this happened when the Word file is converted into PDF or, the resolution of the original plots is not high enough.

Response: Many thanks to the reviewer for recognizing our work. We are sorry for the poor resolution of the figures in the main text. We have replaced them with high-resolution figures and improved the image quality. Thanks again for this comment.

 

Reviewer #2:

2. Some plots, the left side, margin too narrow. Only half of the letters 'a1, b1...' are displayed (for example, Figures 6 & 7).

Response: Thank the reviewer for this comment. We are sorry for this problem. We have reorganized all the plots to make them more clear. Thanks again for this comment.

 

Reviewer #2:

3. What is the source of the data/plot in Figure 2 ? Please cite it.

Response: Thank the reviewer for this comment. We are sorry we missed the reference. In the updated version, we have cited the reference. Thanks again for this comment.

 

Reviewer #2:

4. Some text have larger font size with underlines, this is not allowed in journal papers. Please revise this.

Response: Thank the reviewer for this comment. We are sorry for this mistake, and we have modified the text font to keep uniform and removed the underlines. Thanks again for this comment.  

 

Reviewer #2:

5. lines 130-132,'Since the highly oxidizing environments during the OER may oxidize the lattice oxygen,[46] the stability of the oxygen vacancies is also worth studying.'

 

I think the highly oxidising environment will affect the stability of the oxide or oxide lattice, not the oxygen vacancies.

Response: Thank the reviewer for this comment. During the OER conditions, the oxygen vacancies of perovskite oxides can be refilled by the intake of hydroxyl groups (J.Am.Chem. Soc. 2014, 136, 42, 14646-14649), and the high oxygen vacancy amount present in perovskite oxides demonstrated to trigger the lattice oxygen evolution reaction (Nat. Mater. 2017, 16, 925-931) and the further surface phase transformation. We have changed the sentence into “the stability of highly oxygen-deficient perovskite oxides is also worth studying”. Thanks again for this comment.

 

Reviewer #2:

6. lines 175-176, 'In addition, solid thin films are not suitable for practical application due to the limited number of active sites available.' This statement is incorrect.

Response: Thank the reviewer for this comment. We come to this conclusion based on the following considerations. For practical water electrolyzer electrodes, porosity and large surface areas are required to expose more active sites and avoid gas bubble blockage. However, deposition-prepared thin films are dense and lacking porosity. In addition, the cost of preparing epitaxial thin films by the deposition method (e.g. PLD) is usually high, which further hinders their practical applications. We have modified this sentence into “In addition, it is challenging to apply solid thin films for practical water electrolysis due to the lack of porosity and the high preparation cost”. Thanks again for this comment.

 

Reviewer #2:

7. lines 221-223, 'We should note that supports or substrates are usually required for exsolution-prepared nanoparticles or deposition-derived nanofilms, which potentially limits the number of active sites available.' This is not correct. Exsolution can happen on nanoparticles without substrates.

Response: Thank the reviewer for this comment. We agree with the reviewer substrates are not necessary for exsolution, but the perovskite oxide backbone is still required as the support of the exsolved nanoparticles. We have modified this sentence into “We should note that perovskite oxide supports are usually required for exsolution-prepared nanoparticles, while substrates are necessary for deposition-derived nanofilms”. Thanks again for this comment.

 

Reviewer #2:

8. some language errors, for example,

lines 310-311, 'has been also used' should be 'has also been used'

'simple perovskites' should be 'primitive perovskites'

There are other language errors too.

Response: Thank the reviewer for this comment. In the updated version, “has also been used” is used, and “simple perovskite” has been changed into “primitive perovskite”. The English writing of this manuscript has been checked by Editage (www.editage.co.kr). Thanks again for this comment.  

 

 

Author Response File: Author Response.pdf