Core-Shell ZnO@Cu₂O as Catalyst to Enhance the Electrochemical Reduction of Carbon Dioxide to C₂ Products

Round 1

Reviewer 1 Report

The work is well written and the data are clearly presented. Authors used many different techniques to characterize synthesized material: SEM, XRD, TEM and XPS. Finally, after deposition of the material onto the support, the electrochemical measurements and simultaneously the gas products were detected using gas chromatography. DEspite the whole work is interesting and authors put a lot of efforts to describe the material in details, some corrections are needed as following:

a) In the introduction the novelty should be underlined because now it is not clear what is the differece between the presented results and the current stat of knowledge

b) The distibution shown as an inset in Fig. 1 and 2 is hardly visible and I suggest to put it as an additional figure.

c) On page 4, there is given Cu with 0 and 1+ in superscript that is confusing for the reader. Please, edit this abbreviation in the whole manuscript.

d) Scheme 1 - the figure caption starts with the small letter

e) In the experimental section, the information regarding the amplitude for the EIS measurements is missing.

f) in the conclusions authors wrote that they synthesized a new type of catalyst but some explanation of this novelty is not stated there

g) In the section 3.1 (-1) by cm in a unit of water conductivity is not given in the superscript.

h) regarding analysis of EIS spectra the values of the material resistance should be determined and provided.

Author Response

Response to Reviewer 1’s Comments

 

The work is well written and the data are clearly presented. Authors used many different techniques to characterize synthesized material: SEM, XRD, TEM and XPS. Finally, after deposition of the material onto the support, the electrochemical measurements and simultaneously the gas products were detected using gas chromatography. Despite the whole work is interesting and authors put a lot of efforts to describe the material in details, some corrections are needed as following:

Comment 1: In the introduction the novelty should be underlined because now it is not clear what is the difference between the presented results and the current state of knowledge.

 

Response 1: Thanks for your valuable comment, which is helpful to improve the quality of our manuscript. According to your suggestion, we have checked carefully again and modified the text "Inspired by these works, we designed and synthesized a catalyst with ZnO core and Cu₂O shell through a wet chemical method. Due to the double active sites on the ZnO@4Cu₂O catalyst. It is possible to make CO₂ first generate CO at the ZnO site, and then CO couples on Cu₂O to generate C₂ product. The synergistic effect of Cu and Zn enhances the selectivity of C₂ products on the catalyst. This core-shell structure was used to enhance the multielectron reactions in electrochemical CO₂ reduction in a flow cell. And the thickness of the Cu₂O shell was controlled to boost the coupling of CO on the Cu site to produce C₂ products. By tailoring the synergy of Cu and Zn oxides, high faradic efficiency of 49.8% for C₂ products at a high current density of 140.1 mA cm^-2 was achieved under the potential of -1.0 V vs RHE. This work exceeded the selectivity of C₂ products with many jobs, While achieving a higher current density " in lines 82-92 on page 2 in the revised manuscript.

 

Point 2: The distribution shown as an inset in Fig. 1 and 2 is hardly visible and I suggest to put it as an additional figure.

 

Response 2: Thank you for your reminding. We have reproduced the Figure 1 and Figure 2, and the particle size distribution diagrams have been placed in Figure S7 and Figure S8 of the supporting information.

 

Figure S7. The particle size distribution of (a) ZnO-400, (b) ZnO-600 and (c) ZnO-800.

 

 

Figure S8. The particle size distribution of (a) Cu₂O, (b) 4ZnO@Cu₂O, (c) ZnO@4Cu₂O, and (d) ZnO@20Cu₂O.

 

Point 3: On page 4, there is given Cu with 0 and 1+ in superscript that is confusing for the reader. Please, edit this abbreviation in the whole manuscript.

 

Response 3: Thanks for your valuable comment, which is helpful to improve the quality of our manuscript. We have revised "The Cu XPS spectra of xZnO@yCu₂O catalysts in Figure 3(c) showed two characteristic peaks of Cu 2p_3/2 at binding energies of 933.4 eV and 935.2 eV mainly attributed to Cu^0,1+ and Cu²⁺" into "The Cu XPS spectra of xZnO@yCu₂O catalysts in Figure 3(c) showed two characteristic peaks of Cu 2p_3/2 at binding energies of 933.4 eV and 935.2 eV mainly attributed to Cu⁺ and Cu²⁺" respectively ". Please see lines 163-173 in the revised manuscript.

 

Point 4: Scheme 1 - the figure caption starts with the small letter.

 

Response 4: We were sorry for this careless mistake. According to your suggestion, we have checked carefully again and corrected this sentence into " Scheme 1. Illustration of formation of xZnO@yCu₂O." Please see lines 120 on page 4 in the revised manuscript.

 

Point 5: In the experimental section, the information regarding the amplitude for the EIS measurements is missing.

 

Response 5: Thanks for the reviewer’s professional suggestion. We have provided the test conditions of EIS in section 3.4 : " The electrochemical impedance spectroscopy of the catalyst was performed in the 0.1 M KHCO₃ electrolyte saturated with CO₂ at -1.0 V vs RHE from 0.01 Hz to 10⁵ Hz.". Please see lines 442-444 on page 13 in the revised manuscript.

 

Point 6: in the conclusions authors wrote that they synthesized a new type of catalyst but some explanation of this novelty is not stated there.

 

Response 6: Thanks for the reviewer’s professional and valuable advice. In this work, we developed a core-shell ZnO@Cu₂O electrocatalyst, which could provide dual active sites to enhance the electrochemical reduction of CO₂ to C₂₊ products. According to your suggestion, we have checked carefully again and corrected this sentence into " In summary, we synthesized a new type catalyst with zinc core-copper shell by sol-gel method and epitaxial shell growth method, which can enhance the selectivity of C₂ products through the synergy between Cu-Zn." Please see lines 446-450 on page 13 in the revised manuscript.

 

Point 7: In the section 3.1 (-1) by cm in a unit of water conductivity is not given in the superscript.

 

Response 7: We were sorry for this careless mistake. According to your suggestion, we have checked carefully again and corrected this sentence into "Ultrapure water (18.2 MΩ cm^-1) was homemade by a Millipore system (Milli-Q Advantage A10)." Please see lines 367 on page 12 in the revised manuscript.

 

Point 8: regarding analysis of EIS spectra the values of the material resistance should be determined and provided.

 

Response 8: Thanks for your valuable comment, which is helpful to improve the quality of our manuscript. According to your suggestion, we have added the values of the material resistance in Table S7. See Table S7 in the Supporting Information.

 

Table S7. The resistance values of different catalysts.

Catalysts	ZnO	4ZnO@Cu2O	ZnO@4Cu2O	ZnO@20Cu2O	Cu2O
Resistance (Ω)	10.7	7.3	2.8	8.9	3.8

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The submitted work describes the synthesis of core-shell catalysts composed by a ZnO core and a Cu₂O shell, applied to the reduction of CO₂ to C₂ products. The synthesis provided core shell catalysts of interest which were characterized by SEM, TEM, EDS, XRD and XPS. The authors relate the increased selectivity in C2 products with the interaction between ZnO and Cu₂O and the ratio between (200)/(111) facets, which facilitates the C-C coupling.

Although the obtained faradaic efficiencies are interesting and in line with what already reported in literature, some major concerns are related to this publication from my point of view. Principally, the authors claim that the (200) and (111) facets ratio has a role in the production of C-C bonds. Although, this might be correct, they proposed this based on XRD results. The main issue regards the fact that XRD is a bulk technique and thus gives and indication on the (200) and (111) facets of the whole sample and not of that on the surface. On the opposite, the reduction of CO2 has to occur on the surface of the catalyst where the ratio between the two facets might be different, and not represented by XRD results. I would expect the authors to investigate the surface of the catalyst exposed to the reaction, to justify their hypothesis, for instance, with XPS, which was also used in this work.

Second, the manuscript it is written in a poor English, with many typos and a non-fluent and correct phrasing. Although the meaning is understandable by English speakers, I think the manuscript does not match the English level required for publication in scientific journals and must be rewritten and reviewed.

Some other comments from my side are found below:

Introduction: The meaning of core-shell catalysts, the advantage that they bring to CO₂ electroreduction and the specific reason of their choice over other Cu₂O/ZnO catalysts should be better addressed.

Line 45:  Many works have been carried out on Cu2O because Cu2O can strengthen the C-C coupling process for the formation of C2 products –  references are missing to justify this sentence.

Line 72: Operando Raman spectroscopy showed that CO* binding on Cu sites was modified by Zn²⁺. – A reference is missing to justify this sentence.

Line 134: Why the Zn was not monitored?

Figure 2: From Cu and Zn mapping it seems that most of copper is segregated alone and not in correspondence of a Zn core. I think that image suggests that core shell particles were not created, at least not completely. I would suggest the authors to further comment on that.

Line 185: It is not clear which formula was used to calculate selectivity.

Figure 5: It seems that the increased C2 selectivity at -1V is likely due to the sharp increase in current density, rather than the presence of Cu2O-ZnO interface. Did the author take this in account? What is the reason of such a sharp increase and sudden decrease at -1.2?

Line 321-325: The author claim that a higher (200)/(111) facets ratio corresponds to a higher interface between the two facets. I think this cannot be said using just this information.

In general, the strong interaction between Zn oxide and Cu oxide which is said to enhance catalytic activity should be investigated by other techniques and possibly quantified as it is a key to reactivity and the most important parameter of this work as claimed by authors.

Figure 7 and its discussion: the authors discuss on the difference between ZnO@20Cu2O and Cu2O but not between ZnO@20Cu2O and ZnO@4Cu2O which would be more interesting.

 

Line 354: triamine citrate is not present among the materials. Were the autors referring to ammonium citrate?

Line 361: Please specify the heating ramp used during calcination.

Line 386: Please specify which GDL was used.

Line 394: please specify the range and scan of the XRD analysis.

Line 403: It was not clear why IrO2 was put on the gas diffusion layer. As Ag/AgCl and Pt were used as other electrodes, is IrO2 part of the catalytic system?

In materials and methods: please specify the brands of the instrumentations. Moreover, it is difficult to understand how the electroreduction experiments were carried out. Could the authors further specify, possibly adding a figure that shows the employed setup in the supplementary materials?

In results and conclusion: The Faradaic efficiencies are expressed as % with decimals (e.g. 49.8%). However, the error on the efficiency showed in the figures is in the order of units. What is the accuracy of the reported data?

 

Supplementary materials – Formula (2) and (3): the parameter e was not specified.

Supplementary materials – in formula (2) the equation of faradaic efficiency looks strange. Usually it is expressed as: FE=(nFZ/Q)*100, where n are the moles of products, F is the Faraday constant, Z are the electrons needed to reduce CO2 to the desired product and Q is the total charge (i*t). Could the authors comment on why they used another kind of formula, show the calculations and if the obtained results are the same?

 

Author Response

Response to Reviewer 2’s Comments

The submitted work describes the synthesis of core-shell catalysts composed by a ZnO core and a Cu₂O shell, applied to the reduction of CO₂ to C₂ products. The synthesis provided core shell catalysts of interest which were characterized by SEM, TEM, EDS, XRD and XPS. The authors relate the increased selectivity in C2 products with the interaction between ZnO and Cu₂O and the ratio between (200)/(111) facets, which facilitates the C-C coupling.

Point 1: Although the obtained faradaic efficiencies are interesting and in line with what already reported in literature, some major concerns are related to this publication from my point of view. Principally, the authors claim that the (200) and (111) facets ratio has a role in the production of C-C bonds. Although, this might be correct, they proposed this based on XRD results. The main issue regards the fact that XRD is a bulk technique and thus gives an indication on the (200) and (111) facets of the whole sample and not of that on the surface. On the opposite, the reduction of CO₂ must occur on the surface of the catalyst where the ratio between the two facets might be different, and not represented by XRD results. I would expect the authors to investigate the surface of the catalyst exposed to the reaction, to justify their hypothesis, for instance, with XPS, which was also used in this work.

 

Response 1: Thanks for your valuable comment, which is helpful to improve the quality of our manuscript. Yes, XRD is a bulk technique and thus gives an indication on the (200) and (111) facets of the whole sample and not of that on the surface. Now, some literature uses XRD and Cyclic voltammogram technology to analyze the influence of facets on the surface of metal catalysts [1-4]. However, as for metal oxides, the surface facets distribution is difficult to be determined. Seldom literature to study the facets on the surface of the copper oxide materials. Although the surface facets might be different with the bulk one, but the bulk facet can reflect the surface one. Therefore, recently some literatures used XRD technology to study the effect of the facets of the cuprous oxide nanocatalyst on the electrochemical reduction of CO₂ [5,6]. Qing [5] et al used XRD and HRTEM to determine the different facets of Cu₂O, and then used other characterizations to study the effect of different crystal faces of Cu₂O on the electrochemical reduction of CO₂. They found Cu₂O octahedra expose the {111} crystal plane with coordinated, unsaturated Cu⁺ sites, and thus, are most active in chemisorbing CO and catalyzing CO oxidation. Gao [6] et al. studied the electrocatalytic CO₂ performance of Cu₂O with different crystal planes by DFT calculation method, and found that the interface formed by Cu₂O (200) and (111) crystal planes is conducive to the production of C₂ products. Inspired by these work, we also used XRD technology to analyze the effect of different facets on the electrochemical reduction of CO₂. XPS is a power technology to characterize the chemical state, surface composition of electrocatalyst, but can’t be used to analyze the facet of the catalyst. Therefore, in this work the XRD technology is used to determine the facets distribution.

 

Point 2: The manuscript it is written in a poor English, with many typos and a non-fluent and correct phrasing. Although the meaning is understandable by English speakers, I think the manuscript does not match the English level required for publication in scientific journals and must be rewritten and reviewed.

 

Response 2: Thanks for the reviewer’s valuable comment, which is helpful to improve the quality of our manuscript. We have tried our best to revise our paper and invited some young teachers graduated from US colleges to improve the English of our manuscript. All the mistakes have been corrected. Now we believe that it meets the requirement for English of this journal. Details are shown in the revised manuscript.

 

Some other comments from my side are found below:

Point 3: Introduction: The meaning of core-shell catalysts, the advantage that they bring to CO₂ electroreduction and the specific reason of their choice over other Cu₂O / ZnO catalysts should be better addressed.

 

Response 3: Thank you very much for kindly reviewing our manuscript and valuable comments. According to the recent literature, the CO₂ electroreduction to produce C₂ products generally is determined by two successive vital steps: CO₂ reduces to the key intermediate CO, and then CO undergoes C-C coupling to produce C₂ products. In order to enhance these processes, many structures were investigated. Ke [7] et al. found that the porous nano ribbon CuO catalyst contains many surface defects and high-density grain boundaries, which is the main reason why the selectivity of C₂ products increases while the selectivity of C₁ products is inhibited. The Cu / Au catalyst prepared by Tao [8] has a selectivity of 81% for formic acid. Among these strategies, core-shell structure shows attractive advantages and investigated by many researchers [9,10]. For example, Zhang et al. [9] designed an Ag @ Cu catalyst by a polyol method. Due to its unique structural advantages, the selectivity of ethylene was obviously improved by the synergy between Ag and Cu. O’Mara et al. [10] synthesized an Ag @ Cu core-shell catalyst to generate different active sites in nanoconfined volumes. The architecture of the nanozyme provides the basis for a cascade reaction, which promotes C–C coupling reactions. However, as for the Cu-Zn bimetallic catalysts, seldom work focus on the core-shell structure with ZnO as core and Cu₂O as shell for CO₂RR. Therefore, in this context, the core-shell ZnO@Cu₂O was developed. The core-shell structure catalyst can make CO₂ produce the key intermediate CO at the core-site. During the outward diffusion of CO, C-C coupling can only be carried out on the Cu₂O shell with limited volume to generate C₂ products. ZnO@4Cu₂O catalyst in our work can electrochemically reduce CO₂ to C₂ products efficiently. CO₂ can generate a large amount of CO on the ZnO core firstly, then CO conducts C-C coupling on the Cu₂O site to generate C₂ products. Our work provides new insights for the electrochemical reduction of CO₂ on Zn-Cu core-shell catalysts.

 

Point 4: Line 45:  Many works have been carried out on Cu₂O because Cu₂O can strengthen the C-C coupling process for the formation of C₂ products – references are missing to justify this sentence.

 

Response 4: Thanks for the reviewer’s valuable comment, which is helpful to improve the quality of our manuscript. Many works have been carried out on Cu₂O because Cu₂O can strengthen the C-C coupling process for the formation of C₂ products [11,12].

 

Point 5: Line 72: Operando Raman spectroscopy showed that CO* binding on Cu sites was modified by Zn²⁺. – A reference is missing to justify this sentence.

 

Response 5: Thanks for your valuable comment. This sentence comes from the reference of [13]. We have cited this reference in the sentence.

 

Point 6: Line 134: Why the Zn was not monitored?

 

Response 6: Thanks for the reviewer’s valuable comment, which is helpful to improve the quality of our manuscript. It may be the error of TEM analysis of element content. In a certain test, the atom ratio of Cu and Zn on ZnO@4Cu₂O was 7:1. Therefore, the TEM data is not accurate enough. Here we take the ICP result (Cu / Zn is 4 / 1) as the standard.

 

Point 7: Figure 2: From Cu and Zn mapping it seems that most of copper is segregated alone and not in correspondence of a Zn core. I think that image suggests that core shell particles were not created, at least not completely. I would suggest the authors to further comment on that.

 

Response 7: Thanks for the reviewer’s valuable comment, which is helpful to improve the quality of our manuscript. Because the Cu and Zn mapping is detected using an as-synthesized sample, not the cross-section. It reflects the composition and element distribution in certain thickness. Therefore, Cu and Zn mapping does not show a clear core-shell structure. But in Figure 2f and Figure S1, we have used the EDS Line scan to analyze the particle, which clearly prove the Core-Shell structures of 4ZnO@Cu₂O, ZnO@4Cu₂O, and ZnO@20Cu₂O. It is similar to the literature [9,14,15].

 

Figure 1. TEM image of (a) 4ZnO@Cu₂O (b) ZnO@4Cu₂O and (c) ZnO@20Cu₂O. The line EDX analysis along the arrow (d) 4ZnO@Cu₂O, (e) ZnO@4Cu₂O and (f) ZnO@20Cu₂O.

 

Point 8: Line 185: It is not clear which formula was used to calculate selectivity.

 

Response 8: Thanks for the reviewer’s valuable comment, which is helpful to improve the quality of our manuscript. According to your suggestion, we have added "The Faraday efficiency of the product on all samples is calculated followed the formulas (1), (2) and (3) in the supporting information." into the manuscript. Please see lines 426-427 on page 13 in the revised manuscript.

 

Point 9: Figure 5: It seems that the increased C₂ selectivity at -1V is likely due to the sharp increase in current density, rather than the presence of Cu₂O-ZnO interface. Did the author take this in account? What is the reason of such a sharp increase and sudden decrease at -1.2?

 

Response 9: Thanks for the reviewer’s valuable comment, which is helpful to improve the quality of our manuscript. We believe that the increase in current density is due to the increase of C₂ product selectivity that requires more electron transfer. The product CO, the key intermediate required for C-C coupling, was from ZnO. ZnO-400 has the strongest ability to generate CO at -1.0 V, so the selectivity of C₂ products increases significantly at -1.0 V vs RHE [13]. However, the CO generation rate on ZnO-400 accelerate as the overpotential increases to -1.2V vs RHE, resulting in the generated CO quickly diffusing and away from the electrocatalyst into gas phase. Thus, the intermediate CO* is insufficient and the C-C coupling process is weakened, leading to the reduction of C₂ products selectivity and total current density with the increase of CO selectivity.

 

Figure 2. Faradaic efficiency of products on different catalysts (a) ZnO-400, (b) Cu₂O and (c) ZnO@4Cu₂O. (d) The total current density of products on different catalysts.

 

Point 10: Line 321-325: The author claims that a higher (200) / (111) facets ratio corresponds to a higher interface between the two facets. I think this cannot be said using just this information.

 

Response 10: We were sorry for this careless mistake. As mentioned in Response 1, XRD results can be used to reflect surface facets distribution. According to your suggestion, we have checked carefully again and corrected this sentence into "As shown in Figure 3(b), the ZnO@4Cu₂O catalyst exposed more enhanced joint interface between (200) and (111) facets due to the highest ratio of (200) / (111) facets, which can facilitate the C-C coupling to produce C₂₊ products." Please see lines 339 on page 10 in the revised manuscript. We also said in lines 340-344 that the result is affected by many factors. The increase of C₂ product selectivity was due to many factors, such as the large electrochemically active area on ZnO@4Cu₂O, the smallest charge transfer resistance, and the fastest reaction kinetics. We have modified these in the revised manuscript.

 

Point 11: In general, the strong interaction between Zn oxide and Cu oxide which is said to enhance catalytic activity should be investigated by other techniques and possibly quantified as it is a key to reactivity and the most important parameter of this work as claimed by authors.

 

Response 11: Thanks for the reviewer’s valuable comment, which is helpful to improve the quality of our manuscript. I am sorry that we did not explain clearly. It was analyzed in the manuscript that the characteristic peak of Zn 2p_3/2 in the XPS spectrum is deflected toward higher binding energy and the slight change in the peak position in the XRD spectrum can explain the interaction between Cu and Zn.

 

Figure 3. (a) XRD patterns of xZnO@yCu₂O catalyst. (b) Zn 2p XPS diagrams of different shell thicknesses catalysts.

 

Point 12: Figure 7 and its discussion: the authors discuss on the difference between ZnO@20Cu₂O and Cu₂O but not between ZnO@20Cu₂O and ZnO@4Cu₂O which would be more interesting.

 

Response 12: Thanks for your valuable comment, which is helpful to improve the quality of our manuscript. According to your suggestion, we have added the description of "The C₂ products selectivity on ZnO@20Cu₂O is lower than ZnO@4Cu₂O at high potential (30% vs 49.8%). ZnO@20Cu₂O had too thick Cu₂O shell and the function of ZnO core may be reduced, accounting for less intermediate CO* and inhibition of the C-C coupling process." in Figure 7. Please see lines 287-290 on page 9 in the revised manuscript.

 

Point 13: Line 354: triamine citrate is not present among the materials. Were the authors referring to ammonium citrate?

 

Response 13: We were sorry for this careless mistake. We have changed "triamine citrate" into "ammonium citrate". Please see lines 372 on page 12 in the revised manuscript.

 

Point 14: Line 361: Please specify the heating ramp used during calcination.

 

Response 14: We were sorry for this careless mistake. According to your suggestion, we have added the sentences "the heating rate is 5 °C / min " in the revised manuscript. Please see lines 380 on page 12 in the revised manuscript.

 

Point 15: Line 386: Please specify which GDL was used.

 

Response 15: We were sorry for this careless mistake. The specifications of GDL in this article are the same. All were purchased from Wuhan Gaoshiruilian Company, without any pretreatment.

 

Point 16: Line 394: please specify the range and scan of the XRD analysis.

 

Response 16: Thanks for your valuable comment, which is helpful to improve the quality of our manuscript. According to your suggestion, we have added the range and scan of the XRD analysis in the revised manuscript. The text is "the scanning range is 20 °-90 °, and the scanning speed was 5 ° / min ". Please see lines 412-413 on page 13 in the revised manuscript.

 

Point 17: Line 403: It was not clear why IrO₂ was put on the gas diffusion layer. As Ag / AgCl and Pt were used as other electrodes, is IrO₂ part of the catalytic system?

 

Response 17: Thanks for your valuable comment, which is helpful to improve the quality of our manuscript. The electrochemical CO₂ reduction was carried out in a flow electrolyzer. The prepared GDE was used as the working electrode. GDL (3.5 cm × 1.5 cm) loaded with IrO₂ and Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively. The electrochemical analysis was carried out in an H-Cell. Platinum mesh and Ag / AgCl electrode were used as the counter electrode and the reference electrode. The catalyst-supported carbon paper is used as the working electrode.

 

Point 18: In materials and methods: please specify the brands of the instrumentations. Moreover, it is difficult to understand how the electroreduction experiments were carried out. Could the authors further specify, possibly adding a figure that shows the employed setup in the supplementary materials?

 

Response 18: Thanks for your valuable comment, which is helpful to improve the quality of our manuscript. The brands of the instrument we used were drying oven (DHG-9070A), centrifuges (H2050R), peristaltic pumps (Masterflex), and muffle furnaces (KSL-1000X-M).

Our electrochemical reduction device is a flow cell, as shown in the figure below, which has been added to Fig. S9. of the supporting information.

 

Figure S9. Flow cell used for electrochemical reduction of CO₂.

 

Point 19: In results and conclusion: The Faradaic efficiencies are expressed as % with decimals (e. g. 49.8%). However, the error on the efficiency showed in the figures is in the order of units. What is the accuracy of the reported data?

 

Response 19: Thanks for your valuable comment. Just like the graphs used in many literatures [16,17], our experimental data are the results of multiple measurements, obtained by calculating the average and variance. The error has been minimized and the accuracy has been enhanced. We can see them in the following figures.

 

 

Figure 4. Faradaic efficiency of products on different catalysts (a) Cu₂O, and (b) ZnO@4Cu₂O.

 

 

 

Point 20: Supplementary materials – Formula (2) and (3): the parameter e was not specified.

 

Response 20: We were so sorry for this careless mistake. We have modified it in the supporting information. The e is the electron charge, the value is 1.60×10^-19 C.

 

Point 21: Supplementary materials – in formula (2) the equation of faradaic efficiency looks strange. Usually it is expressed as: FE=(nFZ / Q)*100, where n are the moles of products, F is the Faraday constant, Z are the electrons needed to reduce CO₂ to the desired product and Q is the total charge (i*t). Could the authors comment on why they used another kind of formula, show the calculations and if the obtained results are the same?

 

Response 21: Thanks for your valuable comment, which is helpful to improve the quality of our manuscript. FE=(nFZ / Q) *100 means the same as formula (2) in this supporting information [13, 18]. The N_i in this paper is the number of elementary charges transferred from a product produced in the reaction, and N _total is the total number of elemental charges transferred by the reaction. nFZ=N_i × e, and Q=N _total × e.

 

References

[1] Cui, E. T.; Lu, G. X. Modulating Photogenerated Electron Transfer and Hydrogen Production Rate by Controlling Surface Potential Energy on a Selectively Exposed Pt Facet on Pt/TiO₂ for Enhancing Hydrogen Production. J. Phys. Chem. C. 2013, 117, 26415-26425.

[2] Xia, B. Y.; Wu, H. B.; Wang, X.; Lou, X. W. Highly Concave Platinum Nanoframes with High‐Index Facets and Enhanced Electrocatalytic Properties. Angew. Chem. Int. Edit. 2013, 52, 12337-12340.

[3] Cui, C. H.; Gan, L.; Heggen, M.; Rudi, S.; Strasser, P. Compositional segregation in shaped Pt alloy nanoparticles and their structural behaviour during electrocatalysis. Nat. Mater. 2013, 12, 765-771.

[4] Arulmozhi, N.; Esau, D.; Lamsal, R. P.; Beauchemin, D.; Jerkiewicz, G.  Structural Transformation of Monocrystalline Platinum Electrodes upon Electro-oxidation and Electro-dissolution. ACS Catal. 2018, 8, 6426-6439.

[5] Hua, Q.; Cao, T.; Bao, H. Z.; Jiang, Z. Q.; Huang, W. X. Crystal-Plane-Controlled Surface Chemistry and Catalytic Performance of Surfactant-Free Cu₂O Nanocrystals. ChemSusChem 2013, 6, 1966-1972.

[6] Gao, Y. G.; Wu, Q.; Liang, X. Z.; Wang, Z. Y.; Zheng, Z. K.; Wang, P.; Liu, Y. Y.; Dai, Y.; Whangbo, M. H.; Huang, B. B. Cu₂O Nanoparticles with Both {100} and {111} Facets for Enhancing the Selectivity and Activity of CO₂ Electroreduction to Ethylene. Adv. Sci. 2020, 7, 1902820.

 

[7] Ke F-S, Liu X-C, Wu J, et al. Selective formation of C₂ products from the electrochemical conversion of CO₂ on CuO-derived copper electrodes comprised of nanoporous ribbon arrays. Catalysis Today, 2017, 288, 18-23.

[8] Tao, Z. X.; Wu, Z. S.; Yuan, X. L.; Wu, Y. S.; Wang, H. L. Copper–Gold Interactions Enhancing Formate Production from Electrochemical CO₂ Reduction. ACS Catalysis, 2019, 9(12): 10894-10898.

[9] Chang, Z. Y.; Huo, S. J.; Zhang, W.; Fang, J. H.; Wang, H. L. The Tunable and Highly Selective reduction products on Ag@Cu bimetallic catalysts toward CO₂ electrochemical reduction reaction. J. Phys. Chem. C. 2017, 121, 11368-11379.

[10] O'Mara, P. B.; Wilde, P.; Benedetti, T. M.; Andronescu, C.; Cheong, S.; Gooding, J. J.; Tilley, R. D.; Schuhmann, W. Cascade Reactions in Nanozymes: Spatially Separated Active Sites inside Ag Core-Porous Cu Shell Nanoparticles for Multistep Carbon Dioxide Reduction to Higher Organic Molecules. J. Am. Chem. Soc. 2019, 141, 14093-14097.

[11] Song, H.; Song, J. T.; Kim, B.; Tan, Y. C.; Oh, J. Activation of C₂H₄ reaction pathways in electrochemical CO₂ reduction under low CO₂ partial pressure. Appl. Catal. B-Environ. 2020, 272, 119049.

[12] Kim, D.; Kley, C.S.; Li, Y.; Yang, P. Copper nanoparticle ensembles for selective electroreduction of CO₂ to C₂–C₃ products, Proc. Natl. Acad. Sci., 114 (2017) 10560-10565.

[13] Ren, D.; Gao, J.; Pan, L. F.; Wang, Z. W.; Luo, J. S.; Zakeeruddin, S. M.; Hagfeldt, A.; Gratzel, M. Atomic layer deposition of ZnO on CuO enables selective and efficient electroreduction of carbon dioxide to liquid fuels. Angew. Chem. Int. ED. 2019, 58, 15036-15040.

[14] Zhang, L.; Jing, H.; Boisvert, G.; He, J. Z.; Wang, H. Geometry Control and Optical Tunability of MetalCuprous Oxide Core-Shell Nanoparticles. ACS. Nano. 2012, 6, 3514-3527.

[15] Tsuji, M.; Yamaguchi, D.; Matsunaga, M.; Alam, M. J. Epitaxial growth of Au@Cu core-shell nanocrystals prepared using the PVP-assisted polyol reduction method. Cryst. Growth & Des. 2010, 10, 5129-5135.

[16] Ting, L. R. L.; Pique, O.; Lim, S. Y.; Tanhaei, M.; Calle-Vallejo, F.; Yeo, B. S. Enhancing CO₂ Electroreduction to Ethanol on Copper-Silver Composites by Opening an Alternative Catalytic Pathway. ACS Catal. 2020, 10, 4059-4069.

[17] Dutta, A.; Montiel, I. Z.; Erni, R.; Kiran, K.; Rahaman, M.; Drnec, J.; Broekmann, P. Activation of bimetallic AgCu foam electrocatalysts for ethanol formation from CO₂ by selective Cu oxidation/reduction. Nano Energy. 2020, 68, 104331.

[18] Su, X.S.; Sun, Y.M.; Jin, L.; Zhang, L.; Yang, Y.; Kerns, P.; Liu, B.; Li, S.Z.; He, J. Hierarchically porous Cu/Zn bimetallic catalysts for highly selective CO₂ electroreduction to liquid C₂ products. Appl. Catal. B-Environ. 269 (2020) 118800.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I thank the authors for their response, which opened a nice discussion and were enough to clear my doubts. 

I think the manuscript can now be published. It seems to me English language is still not a the high level required for the journal. Anyway, it thnik this decision should be on the Editors who surely are more qualified than myself in evaluating the English level of the work.