In operando NMR investigations of the aqueous electrolyte chemistry during electrolytic CO2 reduction

The electrolytic reduction of CO2 in aqueous media promises a pathway for the utilization of the green house gas by converting it to base chemicals or building blocks thereof. However, the technology is currently not economically feasible, where one reason lies in insufficient reaction rates and selectivities. Current research of CO2 electrolysis is becoming aware of the importance of the local environment and reactions at the electrodes and their proximity, which can be only assessed under true catalytic conditions, i.e. by in operando techniques. In this work, multinuclear in operando NMR techniques were applied in order to investigate the evolution of the electrolyte chemistry during CO2 electrolysis. The CO2 electroreduction was performed in aqueous NaHCO3 or KHCO3 electrolytes at silver electrodes. Based on 13C and 23Na NMR studies at different magnetic fields, it was found that the dynamic equilibrium of the electrolyte salt in solution, existing as ion pairs and free ions, decelerates with increasingly negative potential. In turn, this equilibrium affects the resupply rate of CO2 to the electrolysis reaction from the electrolyte. Substantiated by relaxation measurements, a mechanism was proposed where stable ion pairs in solution catalyze the bicarbonate dehydration reaction, which may provide a new pathway for improving educt resupply during CO2 electrolysis.

In their manuscript titled: "In operando NMR investigations of the aqueous electrolyte chemistry during electrolytic CO2 reduction" Jovanovic and coworkers describe a series of NMR spectroelectrochemistry experiments probing both 13C and 23Na resonances.The NMR experiments appear to be expertly done, and the modeling seems to be chemically reasonable.The authors find that speciation and solvation are dependent on the applied potential.The provided data generally supports the conclusions; however, there are some concerns outlined below.Ultimately, the work is well done and the contents a valuable addition to the field.It is this reviewer's recommendation that the manuscript be reconsidered for publication in Communications Chemistry after the following revisions have been made.

Comments:
1. Numerous acronyms are used in the main text but are not defined until much later in the Methods section (i.e.OCV, CA, CP, etc.).Defining these in the main text will greatly enhance the readability of the manuscript.2. The authors present a handful of NMR spectra, that focus on a few selected peaks; however, there is very little in terms of raw data.It would be helpful if the authors included a Supporting Information document with raw spectra/fit of inversion recovery experiment showing how T1 values were calculated as well as a few full sweep NMR spectra during each phase of the electrochemical experiment (i.e. at OCV and during CA and CP). 3. In figure 4 the authors assign the upfield resonance to free ions and the downfield component to ion pairs based on electron density arguments.This is likely the case; however, chemical shift arguments for heteronuclear NMR may not always be so straightforward.It would be more rigorous if the authors could support this with literature examples or computations.
Reviewer #2 (Remarks to the Author): In operando NMR investigations of the aqueous electrolyte chemistry during electrolytic CO2 reduction Comments This is an excellent paper where operando 13C and 23Na were employed to investigate the ion speciation and exchange during electrolytic reduction of CO2 from near electrodes to bulk electrolyte solutions when an electric field is applied.The linkage of exchange between the different solvated species of bicarbonate in terms of free ion pairs, solvent associated ion pairs and contact ion pair is quite interesting.The experiments were done with care and interpretations are generally acceptable.Non-invasive, or non-destructive NMR is unique for studying the details of various kinds of ion pairs when in particular combined with T1 and T2 relaxation measurements.The extend of information obtained are impressive.I recommend the publication provided the following minor changes are made.identical conditions, a lower peak separation of the HCO3 -signal is expected at lower field strength."When ppm is converted to Hz, for the same 10Hz a higher B0 gives a reduced ppm.Therefore, the results in Figure 5 do not justify the minimal exchange rate.
Since T1 and T2 values are different between the different peaks at around 161 ppm in the absence of applied electric field, you can use this result as a strong argument that the peak splitting is not due to the B0 inhomogeneity.
In their manuscript titled: "In operando NMR investigations of the aqueous electrolyte chemistry during electrolytic CO2 reduction" Jovanovic and coworkers describe a series of NMR spectroelectrochemistry experiments probing both 13C and 23Na resonances.The NMR experiments appear to be expertly done, and the modeling seems to be chemically reasonable.The authors find that speciation and solvation are dependent on the applied potential.The provided data generally supports the conclusions; however, there are some concerns outlined below.Ultimately, the work is well done and the contents a valuable addition to the field.It is this reviewer's recommendation that the manuscript be reconsidered for publication in Communications Chemistry after the following revisions have been made.
The authors would like to thank the reviewer for the positive review of our work.As detailed below, we agree with your comments, which enabled us to considerably improve our manuscript.

Comments:
1. Numerous acronyms are used in the main text but are not defined until much later in the Methods section (i.e.OCV, CA, CP, etc.).Defining these in the main text will greatly enhance the readability of the manuscript.
Thank you for the remark.The sections of the manuscript were rearranged before submission, which is why the experimental section contains most of the definitions for the acronyms.In the revised manuscript, the full description of the acronyms was added at their first appearance in the text.
2. The authors present a handful of NMR spectra, that focus on a few selected peaks; however, there is very little in terms of raw data.It would be helpful if the authors included a Supporting Information document with raw spectra/fit of inversion recovery experiment showing how T1 values were calculated as well as a few full sweep NMR spectra during each phase of the electrochemical experiment (i.e. at OCV and during CA and CP).
We agree that the original manuscript lacked raw data.In order to keep the main text as streamlined as possible, we decided to not add raw spectra or T1/T2/Texc evaluations in the manuscript itself, but we have added supporting information, which contain various NMR spectra with a considerably broader chemical shift range recorded during OCV, CA and CP for the 13 C experiments at 14.1 T and 9.4 T as well as the 23 Na experiments at 14.1 T. In order to still be able to discern details in the spectra, we decided to show 13 spectra for each stage at a time resolution of 1 hour instead of all 120 spectra, which we recorded 6 minutes apart.In addition, we included all fit plots that were used to evaluate T1, T2 and Texc of the HCO3 -components and the CO2 signal (SI page 1 -11).Finally, we referred to the SI in the main text (page 13 & 14). 4 the authors assign the upfield resonance to free ions and the downfield component to ion pairs based on electron density arguments.This is likely the case; however, chemical shift arguments for heteronuclear NMR may not always be so straightforward.It would be more rigorous if the authors could support this with literature examples or computations.

In figure
Thank you for your suggestion.We agree that this interpretation lacked theoretical validation.We have included DFT calculations of the chemical shift difference between solvated free ions and solvated ion pairs using two different hydration sphere models.
Overall, the theoretical results are in good agreement with the experimental results.A discussion of the DFT calculations can be found in the manuscript on page 6 and the SI in section 2. Davis Thomas Daniel, who performed the DFT calculations, has been added as a co-author.
Reviewer #2 (Remarks to the Author): In operando NMR investigations of the aqueous electrolyte chemistry during electrolytic CO2 reduction Comments This is an excellent paper where operando 13C and 23Na were employed to investigate the ion speciation and exchange during electrolytic reduction of CO2 from near electrodes to bulk electrolyte solutions when an electric field is applied.The linkage of exchange between the different solvated species of bicarbonate in terms of free ion pairs, solvent associated ion pairs and contact ion pair is quite interesting.The experiments were done with care and interpretations are generally acceptable.Non-invasive, or non-destructive NMR is unique for studying the details of various kinds of ion pairs when in particular combined with T1 and T2 relaxation measurements.The extend of information obtained are impressive.I recommend the publication provided the following minor changes are made.
The authors would like to thank the reviewer for the highly positive reception of our manuscript.As detailed below, we have improved the manuscript based on the reviewer's helpful comments and remarks.
1.It is more difficult to read this manuscript at the beginning but then it becomes much easier to read.Please polish the introduction carefully so that the readers can have a good feeling immediately.You can achieve this by polishing the Language in some way.
We improved the language of the introduction throughout.We also included sentences or passages as transition between certain sections in order to improve readability.
2. Page 6/16: "The coalescence point is dependent on the B0 field, as according to eq. ( 12) it is a function of the absolute peak separation in Hz, and not the relative one in ppm.Thus, under otherwise identical conditions, a lower peak separation of the HCO3 -signal is expected at lower field strength."When ppm is converted to Hz, for the same 10Hz a higher B0 gives a reduced ppm.Therefore, the results in Figure 5 do not justify the minimal exchange rate.
Frist, writing this comment we noticed an error in the formula eq. 12, as well as an inconsistency.Correctly, eq. 12 should equal the exchange rate at the coalescence point.In addition,  is the frequency difference in rad/s.As we give the peak separation in Hz, we substituted  by  =   .This results in:

√𝟐
To be precise, this equation is only strictly correct for two exchanging species of equal concentration, but as ion pairs and free ion signal integrals are on the same order of magnitude, eq.12 is still a good approximation, The corresponding calculations were done correctly and are not affected, just the written formula was incorrect.We adjusted eq. 12 in the manuscript and apologize for the inconvenience.
Second, we improved our estimation of the exchange rate between free ions and xSIPs by calculating exchange spectra based of the modified Bloch equations of Gutowsky and Holm.Using these equations, exchange spectra at 9.4 T could be calculated based on the experimental data at 14.1 T, and then compared to the observed exchange spectra at 9.4 T. Based on these considerations, an exchange rate of 15 1/s could be estimated, which is in line with our previous considerations.Nonetheless, we adapted the manuscript and considered the improved estimation method on pages 3 and 7.
Since T1 and T2 values are different between the different peaks at around 161 ppm in the absence of applied electric field, you can use this result as a strong argument that the peak splitting is not due to the B0 inhomogeneity.
Thank you for the comment.We are, however, hesitant to state whether the differences in relaxation time constants can exclude that the appearance of the shoulder (i.e. the green component) is NOT due to a B0 inhomogeneity.Instead, we theorize in our previous publication that the connected electrodes, even in absence of an electric field, may lead to new relaxation pathways in proximity of the electrodes.For example, conduction electrodes in the metal are paramagnetic, and thus solution in the very proximity of the electrode surface may be affected by paramagnetic relaxation.Thus, the observed shoulder at OCV may actually be a combined result of these relaxation effect AND B0 inhomogeneities.