This is clearly a very challenging subject, to quantitatively characterize the evolution of 13 C NMR signals in the presence of conductive metals, electrochemical processes and various sources of noise and artifacts.

I believe, there are many unknowns left in this study, and questions raised by the authors in the manuscript need to be addressed and verified by them in a systematic manner., e.g., the relaxation measurement inconsistencies, unexpected CO2 signal decay during OCV and effects of bubbles on homogeneity and susceptibility. 

If authors promote this cell design as an advantageous one, there should be a clear evidence of that in terms of quantitative data. I suggest a major revision, which should include troubleshooting addressing the questions raised by the authors, may be with the use of simplified cell design.

Assessment criteria during the full review:

The English needs to be improved quite a bit. A number of errors were noted and paper requires extensive proofreading… e.g. “Lorantzian line shape” – line 365;

What is ppb (line 368 ), did authors mean “ppm” or “part per billion” ? It is hard to see that from the spectra.

The units of concentration are M, or mM… (mmol/L would be mM, line 387);

Line 381: to use “was observed”.

Line 13: “dynamics of the bicarbonate electrolyte changes”

Line 35: “chemical and reaction analysis” … not clear wording... is it  “chemical reaction analysis” ?

Line40: “Several experiment setups … were published”

Line 80: references should appear in chronological or alphabetic order.

Line91 to use: “A three-electrode electrolysis cell that fits a standard 5 mm NMR tube has been build.”

Define “iR” in the text.

Line 275: what is “small geometry” ? "compact design " ?

Line 298: “After introducing the …widths became..”

Kind regards

Assessment criteria during the full review

Does the paper address relevant scientific questions within the scope of MR?

YES

Does the paper present novel concepts, ideas, tools, or data? All submitted papers are assumed to report on new observations and/or new theory; there is no need to draw attention to the novelty in title, abstract, or conclusions.

YES

Are substantial conclusions reached?

No

Are the results sufficient to support the interpretations and conclusions?

No

Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists with reasonable effort?

YES

Are numerical data accompanied by error estimates with a description of the methods used to obtain these estimates?

No

For papers reporting molecular dynamics simulations, is it clearly stated how the MD simulations support the experimental evidence and vice versa?

N/A

Do the authors give proper credit to related work and clearly indicate their own new/original contribution?

YES

Does the title clearly reflect the contents of the paper?

YES

Does the abstract provide a concise and complete summary?

YES

Is the overall presentation well-structured and clear?

Is the language fluent and precise?

Yes

Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated?

YES (repetitive statements noticed).

Fig.4 needs to be more clearly presented.

Are the number and quality of references appropriate?

It appears, the number of references is excessive.

Is the amount and quality of the supporting information and supplementary material appropriate?

N/A

This is a manuscript describing wonderful and detailed work showing in operando measurements of the carbon dioxide / carbonate interconversion on an electrode. It is a challenging problem, and it is indeed interesting to see that this reaction can be monitored in this way. I have the following minor comments:

(1) The discussion about the following is really quite unclear. I could not make sense of this from reading the text and looking at the figure: was the sample rotated or not, why? Do you really need mechanical separation? Why?

"To stabilize the sample inside the magnet and to achive a mechanical separation of probe and cell, a dismounted turbine of a 135 magnet lift was fixed on top of the probe. A spinner was attached to the in operando cell, placed inside the turbine and inserted"

(2) The discussion in relationship to Fig. 4 is nice, but I would suggest to add the following references, which have also shown the orientation effects quite nicely: 

https://pubmed.ncbi.nlm.nih.gov/25036296/

https://pubmed.ncbi.nlm.nih.gov/29960130/

This paper also discusses the orientation effect, and demonstrates it in Fig. S1: