Silver-Carbonaceous Microsphere Precursor-Derived Nano-Coral Ag Catalyst for Electrochemical Carbon Dioxide Reduction

Round 1

Reviewer 1 Report

In this paper, X. Li et al. present a coral-like porous Ag catalyst for the electrocatalytic reduction of CO2 The paper could be interesting for the readership of Catalysts since it presents an interesting method for production of efficient nano-based Ag catalysts. However, this work is not overall well presented and needs a deep revision in English language and style and in the overall readability. Therefore, I can recommend publication only after major revisions, according to the comments motioned below:

1) Many sentences are not clear, for example ‘it can be observed that the applied potential start to increase near -1.0 V vs. SCE, but there is no significant increase for Ag foil in current density until -1.3 V vs. SCE'. Which potential starts to increase? Perhaps the authors mean the current density. Please modify the sentence.

Another sentence that is not clear is: ‘For the 20CD-Ag electrode, the FECO remains about 80% within 5 h, but the current density fluctuates obviously’. Why the authors affirm ‘obviously’? Also, I don’t see this fluctuation in Fig. 1d. Please carefully check unclear sentences in the whole text and revise accordingly. 

2) Different sentences in the text have poor scientific soundness, for example ‘It can be seen that the relative intensities of CD-Ag diffraction peaks are positively correlated with the Ag content in Ag/CM precursors’. What is the meaning of ‘positively correlated’?.

Another one: ‘Among the three different CD-Ag catalysts, the coral-like porous structure of 15CD-Ag is constructed by the uniform and tiny Ag particles together with uniform pore size. However, the obvious non-uniform Ag particle and pore size distribution can be found for 10CD-Ag and 20CD-Ag’  Why the non uniform morphology should be obvious? Also, I would say ‘On the other hand’ or 'Conversely' and not ‘However’.  Please revise the sentence.

3) The reported LSV curves (Fig. 1a) show that 20 CD-Ag produce higher current at the same potential with respect to 10 CD-Ag. According to LSV, how is it possible that FE of 10 CD-Ag is higher than that of 20 CD-Ag?

4) The electrocatalytic performance section (2.1) should be presented after the characterization of electrocatalysts (2.2) and not before.

5) How is the FE calculated? The method is not specified in the text.

6) Authors calculate the double layer capacitance values of different samples, but they refer them as ECSA values. To my knowledge, these values should be divided for the specific capacitance, to derive the ECSA. 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The authors developed the coral-like porous Ag catalyst via a facile synthesis method. This catalyst shows a good selectivity toward CO2 reduction with a high Faraday efficient of 90%. The overall work is comprehensive, and I would suggest it for publication with some minor questions addressed.

1. In figure 1, the authors showed that the 15CD-Ag samples generated the highest current density compared with other samples. But this current density seems to be normalized by the geometric area, can the authors also provide the specific activity and mass activity to confirm the 15CD-Ag sample is the optimal?

2. This silver catalyst seems to be quite stable over a 5h stability test. Can the authors provide a TEM figure of the catalyst after the stability test to prove its morphology stability?

3. The authors showed the size of 15CD-Ag sample centered at 40 nm, how about the other two samples (10CD-Ag, 20CD-Ag)?

3. Following the above question, the authors claimed that the highest ECSA of 15CD-Ag sample is resulted from a uniform microstructure, which is the dominant reason for its best performance. Can the authors explain more clearly about the “microculture”? And all these samples are prepared with the same method, how can they be so different in the microstructure that can induce the large difference in ECSAs ?

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Journal: Catalysts

Manuscript Number: catalysts-1673317

Title: Silver-Carbonaceous Microsphere Precursor Derived Nano-Coral Ag Catalyst for Electrochemical Carbon Dioxide Reduction

In this work, carbonaceous microsphere precursors of varied contents of Ag (Ag/CM) were calcinated to synthesize coral-like porous Ag (CD-Ag) catalysts for the electrochemical reduction of CO₂RR to CO. The highest CO₂RR activity is presented by the catalyst derived by the precursor with the intermediate Ag content, exhibiting a stable current density of –6.3 mA cm^-2 and FE_CO­ of about 90% after 5 h at –1.6 V vs. SCE.

The authors have adequately revealed the structure, morphology, chemical composition, and CO₂RR performance of the as-prepared electrocatalysts via physicochemical (XRD, XPS, TEM, and SEM) and electrochemical results (LSV), respectively. However, the article is not recommended under its current state for publication in “Catalysts” mainly because it lacks a thorough discussion of the results and a sufficient correlation of the physicochemical results with the electrochemical behavior. Below, the points requiring revision are presented.

Major points

1. The novel aspect of the work should be highlighted in the abstract and introduction section. Is the novelty based on the coral-like porous morphology of the Ag catalysts, the utilized Ag-based precursor, the examination of varied Ag contents in the precursor, or all the previous points?
2. The contribution of the as-prepared catalyst should be evaluated based on the previous literature. Specifically, the achieved results of the work (current density, FE_CO, stability) should be compared in a Table with similar Ag-based and generally the best electrocatalysts reported in the last few years.
3. It was found that the FE_CO of the 15CD-Ag catalyst exceeds 90% in the range of –1.7 to –1.9 V vs. SCE. Although the FE_CO is indeed high, the overpotential where it is achieved should also be evaluated compared to previously reported achievements (see comment 2) and its practical application prospect.
4. Could the authors explain, based on the specific synthesis steps followed for each case, how the specific morphologies were formed, i.e., the carbon microspheres and the coral-like porous structure of the final catalyst samples? Alternatively, they could verify the morphologies achieved in the current work with previously reported achievements of works utilizing similar synthesis steps and materials.
5. The current density at –1.6 V vs SCE of the stability tests (Figures 1d and S1) is much lower than that achieved in the LSV curves at the same potential for all tested catalysts. Did the authors use fresh electrolyte and deposited catalytic ink before conducting the stability tests? Consecutive measurements can contaminate the electrolyte and harm the quality of the catalytic ink deposited on the electrode. By the way, why does the 20CD-Ag show a fluctuation in the current density and not in the FE during the stability test? The same question, but for the inverse fact, applies for the 10CD-Ag. Could the behavior of these catalysts be correlated with modification in the oxidation states of the catalysts during the stability test?
6. The TEM images of all the tested catalysts should be presented (in the supplementary information) to evaluate the influence of the Ag content of the precursors to the catalyst morphology. Furthermore, they could be exploited to clarify the following ECSA results.
7. Based on the TEM image of the 15CD-Ag, it could be argued that there are several agglomerates on its surface. To what synthesis condition could this fact be related? Could a more efficient synthesis method be applied to obtain a catalyst with similar physicochemical features but with even higher CO₂RR performance, since the absence of agglomerates could lead to higher ECSA. This point should be highlighted by the authors.
8. Generally, Ag presents two additional peaks at degrees higher than 70^o, corresponding to (311) and (222) crystallographic facets. Although it would not significantly differ from the impact observed for the other facets, the whole range of the XRD patterns should be presented to clarify the influence of the Ag content on all crystallographic facets.
9. The authors distinguished the (200) and (111) Ag crystal planes on the surface of the catalysts via HR-TEM. Considering that the electrocatalytic activities of different crystal planes vary, could the authors comment on this observation in terms of the previously presented CO₂RR electrochemical results?
10. The authors should provide the ECSA values (mg/cm²) of all the catalysts for more practical evaluation. By the way, the behavior of the CVs of Ag foil and 15CD-Ag under different scan rates could indicate than Faradaic processes occur, thus their ECSA were not estimated based on the double layer region. This fact could lead to wrong estimations.
11. (Page: 5 of 11, lines:163-176) The argument for explaining the influence of the Ag content in the precursor on the morphology is not clear, and it should additionally be correlated to the TEM images (see comment 6). The authors should try providing a more thorough explanation. Moreover, the revised explanation should additionally explain the elemental compositions derived from the EDS.
12. The authors correlated the higher oxygen content in the catalysts with an enhanced *CO₂^- activation step and thus with higher CO₂RR activity. However, this point does not seem significant, based on the fact that the Ag foil exhibits similar low performance to the 10CD-Ag. Based on the explanation and the results, it could be specifically correlated with the negatively shifted onset potential of the samples compared to Ag foil. The significant factors for the CO₂RR seem to be the morphology (ECSA and porosity) and the chemical surface composition of the catalysts, which should be thoroughly discussed (see the following comment).
13. A porous structure would lead to enhanced diffusion of the reactants on the catalyst surface. Large ECSAs would lead to high performances. Moreover, high Ag⁺ content in the catalyst surface would also lead to enhanced CO₂RR activity. The catalyst order of these properties does not follow the same order as the electrocatalytic activity presented in Figure 1. Furthermore, a modification in the oxidation states of the catalysts could be assumed in the potential window at which the CO2RR was tested, which could explain the dominance of the CO2RR and HER at each potential range, as well as the stability behavior. Based on these blurred points, it is highly suggested to clearly correlate the physicochemical results with the electrochemical and stability results and thoroughly discuss them. Thereafter, possible mechanism pathways should be proposed by providing reaction equations or a scheme and discussing the rate-determining step and the dominance of each physicochemical property on each reaction step.     

Minor points  

• The reported results in the abstract have not been corresponded to a potential (–1.6 V vs. SCE).
• For comparison reasons, it would be advantageous to correct the reference potential of the electrochemical results versus the RHE.
• According to the Catalysts guide for authors, the “materials and methods” section does not precede but follows the conclusion section.
• There are several grammatical and syntactical errors throughout the manuscript. The authors should carefully double-check the text and correct where it is necessary.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I appreciate the authors' efforts in improving the quality of the manuscript. Appropriate changes have been made and now the manuscript is significantly improved in clearness of presentation and scientific soundness. Therefore I can now recommend the acceptance of the manuscript.

Reviewer 3 Report

The authors took into consideration my suggestions and appropriately improved their manuscript. In my opinion it could now be accepted for publication to CATALYSTS.