Tuning the Electronic Properties of Homoleptic Silver(I) bis-BIAN Complexes towards Efficient Electrocatalytic CO₂ Reduction

Round 1

Reviewer 1 Report

In this submission, the authors report the synthesis of six homoleptic silver(I) bis-BIAN complexes and investigate their catalytic performance toward CO₂ reduction. The characterization of these compounds has been systematically carried out. The electrochemical results show that the electron donation of the ligand could increase the activity of the complexes. The heterogeneous catalysis of the complexes was also studied for their potential industrial applications. This topic could attract a wide readership from researchers in the area of catalysis. Therefore, I recommend its publication after addressing the following issues. 

1. XPS data could probe the electron donation of the organic ligands to the Ag ions and may thus be useful to provide more insight into the performance trend of Ag1-Ag6.
2. For the CV result shown in Fig. 2, why do the authors use difference potential range?
3. How about the catalytic stability of Ag6 for CO₂ reduction. 10 cycles as shown in Fig. 4 are not enough.
4. Why do the authors choose Ag4 for the heterogeneous catalysis. Ag6 has much higher catalytic activity than Ag4.
5. Relevant literatures should be cited such as Electrochim. Acta 2020, 342, 136118; ACS Appl. Nano Mater. 2020, 3, 11416−11425; ACS Appl. Energy Mater. 2019, 2, 8851−8861.

 

  

Comments for author File: Comments.pdf

Author Response

Comments and Suggestions for Authors

In this submission, the authors report the synthesis of six homoleptic silver(I) bis-BIAN complexes and investigate their catalytic performance toward CO₂ reduction. The characterization of these compounds has been systematically carried out. The electrochemical results show that the electron donation of the ligand could increase the activity of the complexes. The heterogeneous catalysis of the complexes was also studied for their potential industrial applications. This topic could attract a wide readership from researchers in the area of catalysis. Therefore, I recommend its publication after addressing the following issues. 

 

1. XPS data could probe the electron donation of the organic ligands to the Ag ions and may thus be useful to provide more insight into the performance trend of Ag1-Ag6.

We want to express our gratitude for this interesting suggestion. We have now measured XPS for the complexes Ag1-Ag6, however, the observed N 1s binding energies are within a particularly small margin, without displaying a clear trend. The CF₃ substituted Ag1 exhibits the lowest Ag 3d binding energies caused by the strongly electron withdrawing effect of the pertinent CF₃ moieties. The data obtained for Ag2 exhibit the highest Ag 3d binding energies caused by the electron donating isopropyl groups attached at the ortho-positions of the aromatic substituent. These XPS data suggest that substitution on the ortho- positions of the Aryl-BIAN has the strongest influence on the observed Ag binding energies. Despite these two observations, again no clear trend for the other investigated complexes can be found. As a result, an in-depth discussion thereof seems to be beyond the scope of this study. Nevertheless, the recorded spectra are now included in the supporting information as Figures S 62 and S 63.

1. For the CV result shown in Fig. 2, why do the authors use difference potential range?

Thank you for pointing this out, the CV for [Ag(ACN)₄]BF₄ was again measured to fit with the displayed potential ranges of L6 and Ag6 and is now depicted in line 186 in the revised Figure 2 (additionally depicted below).

 

Figure 2: CV response overlay of 1mM [Ag(ACN)₄]BF₄, BIAN ligand L6 and Ag(I) bis-BIAN complex Ag6 in ACN under argon at 100 mV s^‑1 scan rate.

1. How about the catalytic stability of Ag6 for CO₂ reduction. 10 cycles as shown in Fig. 4 are not enough.

As described in more detail for remark 4, Ag6 has shown diminished catalytic activity upon increasing the water contents of the investigated solvent mixtures. Since the intention of this research was to provide novel potent materials for sustainable electrochemical conversion of CO₂ we focused on their applicability in purely aqueous electrolytes, for the most environmentally benign mode of application. Therefore, Ag4 has been proven to be better suited and long-term stability of Ag6 has not been in the focus of further research. Since CVs of the bare ligand L6 and the silver precursor [Ag(ACN)₄]BF₄ have shown substantial decreases of the detected cathodic currents under CO₂ already after the first cycle (as discussed in 247-262), indicating either rapid decomposition or only stochiometric reactions of the former with carbon dioxide. For this purpose, Figure 4D is supposed to serve as illustrative proof that the investigated silver BIAN complexes do not feature this problematic behavior.

1. Why do the authors choose Ag4 for the heterogeneous catalysis. Ag6 has much higher catalytic activity than Ag4.

An important question which we have discussed in lines 201‑207, 271-276 and 291-294. The catalytic activity of Ag6 is superior in acetonitrile + 2% water mixtures compared to the other catalysts. However, upon increasing the water content up to 20%, the obtained currents under CO₂ atmosphere of Ag4 and Ag6 were already nearly identical (271-276). The issue with Ag6 is the rather basic amino-moiety in the BIAN backbone, which leads to at least partial protonation thereof in aqueous solutions, resulting in the loss of the +M-effect and consequently the electron donating (catalytically activating) properties (201-207). This effect was expected to be even more pronounced in purely aqueous electrolytes, therefore Ag4 was chosen for heterogeneous catalysis.

1. Relevant literatures should be cited such as Electrochim. Acta 2020, 342, 136118; ACS Appl. Nano Mater. 2020, 3, 11416−11425; ACS Appl. Energy Mater. 2019, 2, 8851−8861.

We are grateful for recommending these interesting pieces of literature. The introduction has been revised and the suggested publications as well as additional recent literature is now cited

Reviewer 2 Report

The preparation and characterization of six readily assembled bis-coordinated homoleptic silver(I) N,N′-bis(arylimino)acenaphthene (BIAN) complexes of the general structure [Ag(I)(BIAN)2]BF4 was reported. The electrochemical performance of CO2 reduction was investigated, all catalysts showed improved current and no significant H2 evolution. The author may address following comments/questions to improve the manuscript.

1. Introduction, 32-33 and 38-39. Need to put citation(s) at each example (H2O, O2, N2, etc)
2. 151-152; can author provide the table about the potential
3. 155-158, can author explain why there is different potential for Ag (x)
4. 196: how the author knows the current is not from hydrogen evolution of the addition of H2O. If not, can author explain why the addition of water increase the activity?
5. 294-295: the author needs to include the GC MS results to demonstrate the product is CO. May include that in SI.
6. For the stability test: can author explain why to choose 10 cycles? And what is the main root cause for the catalysis lose in CO2 reduction? The author need to add some discussion of stability at the introduction part.

Author Response

Comments and Suggestions for Authors

The preparation and characterization of six readily assembled bis-coordinated homoleptic silver(I) N,N′-bis(arylimino)acenaphthene (BIAN) complexes of the general structure [Ag(I)(BIAN)2]BF4 was reported. The electrochemical performance of CO2 reduction was investigated, all catalysts showed improved current and no significant H2 evolution. The author may address following comments/questions to improve the manuscript.

 

1. Introduction, 32-33 and 38-39. Need to put citation(s) at each example (H2O, O2, N2, etc)

We are grateful for your comment. We have addressed this issue in the revised manuscript and included additional recent literature in the introduction.

1. 151-152; can author provide the table about the potential

A table with the determined Fc/Fc⁺ half wave potentials (Table S 2, additionally shown below) has been added to the supporting information and is now referenced 156. The varying potentials are due to the different Ag/AgCl QREs employed in each mixture, which is the reason we determined the Fc/Fc⁺ potential after each individual measurement.

 

	E1/2 / V vs Ag/AgCl QRE
Compound	ACN	ACN + 2% H2O	ACN + 20% H2O
L4	0.403	-	-
L6	0.448	0.419	-
Ag(ACN)4BF4	0.140	0.182	0.018
Ag1a	0.137	0.084	-
Ag2	0.204	0.192	-
Ag3	0.189	0.178	-
Ag4	0.200	0.193	0.129
Ag5	0.194	0.170	-
Ag6	0.239	0.239	0.163

 

1. 155-158, can author explain why there is different potential for Ag (x)

Unfortunately, there is no clear trend in the observed potentials, which is coherent with what is reported in the literature for similar BIAN complexes (Eur. J. Inorg. Chem. 2013, 5196–5205, Eur. J. Inorg. Chem. 2013, 2418–2431). While the most electron deficient complex Ag1 is in fact the easiest to reduce (lowest applied negative potentials for reduction) this tendency is not explicitly expandable onto the other explored complexes. This circumstance makes the precise interpretation and explanation of this data ambigous, as they are most likely influenced by a combination of electronic and steric components of the employed ligands. For this purpose, we wanted to refrain from making assumptions and therefore abstained from a deeper discussion of these properties.

1. 196: how the author knows the current is not from hydrogen evolution of the addition of H2O. If not, can author explain why the addition of water increase the activity?

Truly an important question. Since we also measured CVs for the mixtures of ACN +2%/20% H₂O under argon and did not observe an increase in cathodic current up to ca. -2V vs NHE, as discussed in 234-236 and 265-269, H₂ evolution by virtue of water splitting can effectively be ruled out for these catalytic systems. As mentioned in 226-233, proton sources, such as H₂O, allow for different reduction mechanisms/products and hence distinct kinetics which, in this case, lead to the reported increase in catalytic currents.

1. 294-295: the author needs to include the GC MS results to demonstrate the product is CO. May include that in SI.

Thank you for pointing this out. GC-MS chromatograms are now included in the supporting information as Figure S 107 – S 109 for all measured current densities (50, 100 and 200 mA cm^-2).

1. For the stability test: can author explain why to choose 10 cycles? And what is the main root cause for the catalysis lose in CO2 reduction? The author need to add some discussion of stability at the introduction part

The main reason for scanning multiple cycles was to determine whether the detected current was in fact stemming from a catalytic reaction at the electrogenerated species, or if it was originating from a stochiometric side reaction/decomposition reaction of some sort. This was, as discussed in 247-262, the case for the pure ligand L6 and the silver precursor [Ag(ACN)₄]BF₄ as shown in Figures S 76 and S 85, detectable starting after the first cycle, with substantial decreases of the measured cathodic currents. 10 Cycles were therefore chosen to be able to descriptively demonstrate with a factual measurement that this is not the case for the investigated silver BIAN complexes (Figure 4D). Long term stability tests were as it were incorporated in the heterogeneous electrochemical measurements, or respectively the determined stable half-cell potentials shown in Figure 5B, establishing the stability and applicability of the silver complexes. Determination of a deactivation mechanism was beyond the scope of this work, as such deactivation was not observed for the explored silver BIAN catalysts.

We want to express our gratitude for this comment, we have now added short discussions regarding the stability in the introduction (67-70) and the section of heterogeneous electrochemistry (299-301).