Effects of ligand tuning and core doping of atomically precise copper nanoclusters on CO2 electroreduction selectivity

Atomically precise nanoclusters (NCs) provide opportunities for correlating the structure and electrocatalytic properties at atomic level. Herein, we report the single-atom doping effect and ligand effect on CO2 electroreduction (eCO2RR) by comparing monogold-doped Au1Cu24 and homocopper Cu25 NCs protected by triphenylphosphine or/and tris(4-fluorophenyl)phosphine. Catalytic results revealed that the electronic distribution of Cu25 NCs is enormously contracted by doping Au atoms, entitling it to exhibit the unique inhibition of hydrogen evolution reaction. And the inductive effect of ligand strongly favors the formation of formate in eCO2RR. Overall, this work will provide guidance for the rational design of the copper-based catalysts in the eCO2RR.

Reviewer #2 (Remarks to the Author): Recommendation: May be suitable for Communications Chemistry after small revisions. Comments: In this work, the authors presented a case for the impact of electron configuration modulating over atomically precise copper nanocluster on electroreduction selectivity. They also found that the four clusters exhibited different catalytic performances toward CO2RR, in which single-atom doping clusters converts CO2 into CO and formate, while hydrogen evolution reaction are the main reduction prossess for homocopper Cu25 NCs. And the inductive effect of ligand strongly favors the formation of formate in CO2RR. Overall, it is a high quality work. This manuscript is recommended for publication if the following concerns can be addressed: 1) Please describe the FE calculation method in detail, including the collected method for gas products.
2) The four clusters were respectively deposited onto acid multi-walled carbon nanotubes support, I want to understand whether there is an interaction between nanoclusters and support. Why did the author use carbon nanotubes rather than carbon black? 3) Furthermore, the stability of the nanoclusters before and after electrocatalysis was characterized by UV-vis spectra ( Figure S9). I want to know what causes the subtle differences of absorbance profile of nanoclusters before and after electrocatalysis? 4) There are some small mistakes in the current manuscripts as follow: a. The description of Figure S7 is incorrect. b. It is better to make the abbreviation of CO2 electroreduction (eCO2RR) unified. c. The author should point out the test potential of the 1H-NMR spectra of the eCO2RR product in liquid phase for the four clusters.
Reviewer #3 (Remarks to the Author): Recommendation: Minor revision Comments: Mei Ding and coworkers present that the singe-atom doping effect and inductive effect on CO2 reduction by comparing with four M@Cu24 nanoclusters (protected by triphenylphosphine or tris(4fluorophenyl)phosphine) that share a similar M@Cu12 core structure, and the results demonstrate that single-atom substitution can substantially improve CO2 electroreduction activity and selectivity. The authors did a good job on explaining the production method and investigating the impact of electron configuration modulating on CO2 reduction. The story is well-organized. My recommendation is minor revision. There are some minor issues must be addressed prior to publish as an article in Communications Chemistry: 1. The stability is one of the important indexes to evaluate the catalyst material. The authors provide the stability test of four clusters at -0.8 V vs. RHE for 5 h. I want to know why did the author chose the stability test of four clusters at -0.8 V? 2. The resolution of figures in supporting information is too low to be seen clearly. It is better to improve the definition of the figures to clearly express subtle differences. 3. There are some small mistakes and typos. E. g. expression of sentence, such as " Figure S2b" should be " Figure S2c"; the "NC" should be "NCs"; there is lack of "=" in equation of S3; journal names should be correctly in reference; the "Cupric (II) acetylacetonate" should be "cupric (II) acetylacetonate"; "Synthesis of [Cu25H22(Ph3P)12]+ nanocluster" should be "Synthesis of [Cu25H22(Ph3P)12]+ nanoclusters" The authors must check the whole manuscript carefully to make sure to avoid these errors. In summary, this manuscript currently has basically satisfied the high scientific standard and high impact demand of Communications Chemistry, hence a minor revision is recommended at this stage. After the above issue have been unambiguously addressed, it can be considered to publish in Communications Chemistry.

/ 10
We thank the reviewers for the constructive comments. We have addressed all the comments point-topoint unambiguously and made corresponding changes in the text. The relevant texts are highlighted in red in the revised manuscript and supporting information. The comments are in blue and the responses are in black listed as follows.

Reviewer #1:
The manuscript by Ding et al. reports single-atom doping effect and ligand effect on CO2 electroreduction (CO2RR) by comparing monogold-doped Au1Cu24 and homocopper Cu25 nanoclusters protected by triphenylphosphine or/and tris(4-fluorophenyl)phosphine. Several issues need to be further clarified. The manuscript can only be considered after the authors address the following points. Q1: Compared with Figure 2, the partial current density in Figure S7 seems to miss the error bar?
And the equation S3 makes a mistake that "=" missing?
Response: Thanks a lot. We have replaced original Figure S7 with partial current density with error bar in new Figure S7. And we have corrected the mistakes in equation S3. Furthermore, we have carefully checked and corrected all mistakes like this in the whole manuscript. Q2: What is the different between Qt (the total charge consumed) and C (total charge consumed in electrochemical reaction)? How to calculate them?

Response:
We are truly sorry for that. Both Qt and C are defined as the total amount of electricity consumed in electrochemical reactions, which were recorded by electrochemical workstation. Therefore, we have removed "C" to ensure uniform expression. Furthermore, we have carefully checked and corrected all the small mistakes in the whole manuscript.
Q3: The 5-h long-term stability measurement is too short, please prolong it and add the corresponding FEs.
Response: Thanks for the kind comment. We have prolonged the time of stability measurement to 12 h. And we have re-drawn the Figure 3 and added corresponding descriptions in the revised manuscript. Concretely, as illustrated in Figure 3a and 3d, the current density and corresponding FE value of two mono-Au doped NCs remained almost unchanged (jtotal= -7.13 to -6.91 mA·cm -2 for AuCu24- Concretely, no nanoparticles were observed on the surface of four electrocatalysts, which corresponds to the X-ray diffraction (XRD) pattern ( Figure S5). Furthermore, the electrochemical impedance (EIS) was carried out to explore the charge transport properties of the four NCs at the electrode/electrolyte interface ( Figure S12). The Nyquist plot of both homogeneous Cu25 NCs exhibited a much smaller the semicircular diameters than that of mono-gold-doped AuCu24, and the former has a conductivity with lower interfacial charge-transfer resistance.

Reviewer #2
Recommendation: May be suitable for Communications Chemistry after small revisions. Comments: In this work, the authors presented a case for the impact of electron configuration modulating over atomically precise copper nanocluster on electroreduction selectivity. They also found that the four clusters exhibited different catalytic performances toward CO2RR, in which single-atom doping clusters converts CO2 into CO and formate, while hydrogen evolution reaction are the main reduction prossess for homocopper Cu25 NCs. And the inductive effect of ligand strongly favors the formation of formate in CO2RR. Overall, it is a high quality work. This manuscript is recommended for publication if the following concerns can be addressed: 1) Please describe the FE calculation method in detail, including the collected method for gas products.
Response: Thanks for the comment. We have added the details of FE calculation in the Electrochemical measurements and marked in red. The output of the gas flow from the cathode chamber didn't be collected by us, while it was in situ online directed into a gas chromatograph instrument (GC3900Plus, RUI NENG) for identification and quantification of the gaseous products, which was purged for 30 min with an average rate of 10 mL·min -1 (at room temperature and ambient pressure) prior to the test.
2) The four clusters were respectively deposited onto acid multi-walled carbon nanotubes support, I want to understand whether there is an interaction between nanoclusters and support. Why did the author use carbon nanotubes rather than carbon black?
Response: Thanks a lot. Due to the small size of the clusters (size <3 nm), they can be stably bound to carbon nanotubes by physical adsorption. Compared with carbon black, the surface of acid multiwalled carbon nanotubes possesses much more defects, which can contribute to CO2 enrichment.
Meanwhile, the holes between carbon fibers can increase CO2 penetration, which in turn improves the contact between the catalyst and CO2. Therefore, we have chosen acid multi-walled carbon nanotubes as conductive support rather than carbon black.
3) Furthermore, the stability of the nanoclusters before and after electrocatalysis was characterized by UV-vis spectra ( Figure S9). I want to know what causes the subtle differences of absorbance profile of nanoclusters before and after electrocatalysis?