Bifunctional CuO-Ag/KB Catalyst for the Electrochemical Reduction of CO₂ in an Alkaline Solid-State Electrolysis Cell

Round 1

Reviewer 1 Report

Interesting manuscript on electrochemical reduction of CO₂ using a bifunctional CuO-Ag/KB catalyst in an alkaline solid-state electrolysis cell. The methodology and results provided are interesting, however, it needs to be revised before this manuscript can become suitable for publication. My questions/suggestions are as below.

My questions/suggestions

1. Define the acronym ‘CRM’ in the introduction were first mentioned (line 33) and not (line 44).
2. In the materials and methods, the authors mention using a reducing gas (5%H₂/He) to form Ag metallic species, does it not affect CuO?
3. In the manuscript, the authors show XRD for individual Ag and CuO. Why not show an XRD for the combined CuO-Ag catalyst?
4. Authors do not compare their results with relevant literature as to what yields of methanol and ethanol have been obtained on comparable catalysts or conditions?
5. The authors also mention SEM-EDX characterization in the materials and methods but do not show the results from that. Any reason?
6. The authors mention lower deactivation in the case of CuO-Ag as compared to CuO. Is it possible to do any post characterization of these materials? other than the reaction data.

Author Response

We thank the reviewer for valuable observations and suggestions for improving the manuscript. This document reports our viewpoint regarding some of their observations and our point-by-point replies. The revisions made to the manuscript are highlighted in yellow.

Reviewer #1:Interesting manuscript on electrochemical reduction of CO₂ using a bifunctional CuO-Ag/KB catalyst in an alkaline solid-state electrolysis cell. The methodology and results provided are interesting, however, it needs to be revised before this manuscript can become suitable for publication. My questions/suggestions are as below.

My questions/suggestions

1. Define the acronym ‘CRM’ in the introduction were first mentioned (line 33) and not (line 44).

1. We thank the reviwer for this observation. We defined this acronym in the “introduction” section (page 1).

2. In the materials and methods, the authors mention using a reducing gas (5%H₂/He) to form Ag metallic species, does it not affect CuO?

1. We thank the reviewer for this comment. We clarified this aspect in the revised version of the manuscript (page 9, lines 208-209).

3. In the manuscript, the authors show XRD for individual Ag and CuO. Why not show an XRD for the combined CuO-Ag catalyst?

1. In the revised version of the manuscript (pages 2-4) other physico-chemical characterizations of the combined CuO-Ag catalyst, such as XPS, XRD and SEM-EDX, were added (Fig.s 1-4).

4. Authors do not compare their results with relevant literature as to what yields of methanol and ethanol have been obtained on comparable catalysts or conditions?

1. Thank you for this observation. We have improved the literature and included some discussion (page 8, lines 188-190 and lines 199-202).

5. The authors also mention SEM-EDX characterisation in the materials and methods but do not show the results from that. Any reason?

1. Thank you for this observation; we have improved the text accordingly in the revised version of the manuscript (page 3-4, lines 92-98)

6. The authors mention lower deactivation in the case of CuO-Ag as compared to CuO. Is it possible to do any post characterisation of these materials? other than the reaction data.

Thank you for this observation. Currently, we have the TEM equipment in the maintenance and we can not observe the microscopic properties of the spent specimen. We carried out the XRD spectrum of spent catalyst, but we would avoid showing this analysis on the main text due to the complexity caused by the presence of the ionomer. In general, we recognised the presence of AgOx that is quite normal to be formed under the operational conditions described in the paper (i.e. pH and potentials). There were no traces of cuprite, and copper was mainly as tenorite phase.

Reviewer 2 Report

Recommendation: major revision.

Comments: In this work, the authors report a bifunctional CuO-Ag/KB catalyst for the electrochemical reduction of CO₂ in an alkaline solid-state electrolysis cell. The experimental is reasonably designed and the electrochemical performance is good. The experiment data relevant to the CuO-Ag/KB offered in this manuscript are sufficient to support the conclusion. But, the structural stability of these CuO-Ag/KB catalysts needs more data to support. Therefore, I recommend that this manuscript major revision.

1. The XRD characterization of CuO-Ag/KB catalyst after tests may add to investigate the structuralstability thiscatalyst undergoes during the CO₂ reduction process.
2. It is well known that Ag is a good catalyst for CO₂reduction and the main product is CO. So, all the CO₂ reduction products and corresponding Faradic efficiency of CuO-Ag/KB catalyst are better offered.
3. The authors should compare the performance with reportedCuO-based catalysts. 
4. There are some writing mistakes in the manuscript. The authors should carefully check and correct them.

Author Response

We thank the reviewers for their valuable observations and suggestions for improving the manuscript.

This document reports our viewpoint regarding some of their observations and our point-by-point replies. The revisions made to the manuscript are highlighted in yellow.

Reviewer #2: In this work, the authors report a bifunctional CuO-Ag/KB catalyst for the electrochemical reduction of CO₂ in an alkaline solid-state electrolysis cell. The experimental is reasonably designed and the electrochemical performance is good. The experiment data relevant to the CuO-Ag/KB offered in this manuscript are sufficient to support the conclusion. But, the structural stability of these CuO-Ag/KB catalysts needs more data to support. Therefore, I recommend that this manuscript major revision.

1. The XRD characterisation of CuO-Ag/KB catalyst after tests may add to investigate the structural stability this catalyst undergoes during the CO₂reduction process. 

Thank you for this observation. Currently, we have the TEM equipment in the maintenance and we can not observe the microscopic properties of the spent specimen. We carried out the XRD spectrum of spent catalyst, but we would avoid showing this analysis on the main text due to the complexity caused by the presence of the different phases species like as KOH and ionomer. In general, we recognised the presence of AgOx that is quite normal to be formed under the operational conditions described in the paper (i.e. pH and potentials).. There were no traces of cuprite, and copper was mainly as tenorite phase.

2. It is well known that Ag is a good catalyst for CO₂ reduction and the main product is CO. So, all the CO₂reduction products and corresponding Faradic efficiency of CuO-Ag/KB catalyst are better offered.

1. We agree with this observation and in the revised text we discussed this point at page 6, lines 158-160

 The authors should compare the performance with reported CuO-based catalysts. 

3. We thank the referee for this comment that gave us the possibility of further clarification about the role of this paper in the advancement of knowledge. We included some additional references in the text, and we discussed our results according to the literature. We noticed as most research activities in this field are based on semicell studies. In our paper, we investigated in depth the physico-chemical properties of a benchmarked electrocatalyst formula but prepared in our labs and reported the electrochemical behaviour in a complete cell configuration and the analyses of outlet residual liquid phase.

4. There are some writing mistakes in the manuscript. The authors should carefully check and correct them.
5. Yes, we did an overall revision of the text

Reviewer 3 Report

In this work, Zignani et al. present a study about bifunctional CuO-Ag/KB catalyst for the electrochemical reduction of CO2 to fine materials in an alkaline solid-state electrolysis cell. I think the subject of this manuscript is interesting but the manuscript needs some major modifications.

1. Figure 1 and Figure 2 should be merged.
2. TEM images should be provided at different magnification (for example 5 nm, 50 nm, and 500 nm).
3. XPS and EDS spectra as well as BET spectrum should be provided.
4. What is the significance of this paper over other related papers? The authors should clarify the benefits of their work. In other words, is really there any difference between their work and previously published articles? The answer to this question will determine the final decision for accepting the article.

Author Response

We thank the reviewers for their valuable observations and suggestions for improving the manuscript.

This document reports our viewpoint regarding some of their observations and our point-by-point replies. The revisions made to the manuscript are highlighted in yellow.

Reviewer #3: In this work, Zignani et al. present a study about bifunctional CuO-Ag/KB catalyst for the electrochemical reduction of CO2 to fine materials in an alkaline solid-state electrolysis cell. I think the subject of this manuscript is interesting but the manuscript needs some major modifications.

1.Figure 1 and Figure 2 should be merged.

Thank you for this observation. We revised the manuscript accordingly.

2.TEM images should be provided at different magnification (for example 5 nm, 50 nm, and 500 nm).

 We reported in the main text the TEM of fresh CuO catalyst (figure 2). Currently, we have the TEM equipment in the maintenance and we can not observe the microscopic properties of milled catalyst.

3. XPS and EDS spectra as well as BET spectrum should be provided.

Thank you so much for this sugegstion. The revised manuscript now includes XPS and EDX analyses (page 4)

4. What is the significance of this paper over other related papers? The authors should clarify the benefits of their work. In other words, is really there any difference between their work and previously published articles? The answer to this question will determine the final decision for accepting the article.

We thank the referee for this comment that gave us the possibility of further clarification about the role of this paper in the advancement of knowledge. We included some additional references in the text, and we discussed our results according to the literature. We noticed as most research activities in this field are based on semicell studies. In our paper, we investigated in depth the physico-chemical properties of a benchmarked electrocatalyst formula but prepared in our labs and reported the electrochemical behaviour in a complete alkaline cell configuration and the analyses of outlet residual liquid phase.

Round 2

Reviewer 2 Report

I am satisfied with the modification made to this manuscript, which can be accepted now.

Reviewer 3 Report

The current version of the article has been prepared well.