Autocatalyzed Kinetics of 6-Electron Electroreduction of Iodic Acid Studied by Rotating Disk Electrode Technique
Yang Luo
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis manuscript deals with the IO3- reduction reaction kinetics understanding by rotating disk electrode technique, which might be used in iodine-based redox flow batteries. This work can be accepted if the points below are clarified.
1. What are the reasons behind the very low rotating speed (10 rpm, 50 rpm) RDE experiments?
2. The deviation from Levich’s dependence is so large from 11% to 40%. Why? Do you try to fit the data with Koutecký–Levich dependence (ω-0.5-Ilim) considering the Koutecký–Levich dependence was first derived for a chemical-electrochemical reaction (Levich, V. G. Physicochemical Hydrodynamics; Prentice-Hall, Inc., 1962; pp 345−357)?
3. You mention that the iodine layer is thin. Could you provide the exact thinness of the layer after the RDE by some instruments, say SEM?
Minor points:
1. The quality of all figures in the main article can be increased.
2. Line 194: The Figure 1. should be Figure 2.
3. Line 197: Measurements in 0.01M HIO3 – 0.5M H2SO4 should be Measurements in 0.05 M HIO3 – 1 M H2SO4
4. Line 194−Line 197: The SPACE is needed between the numerical value and unit symbol. Please also check other parts of this manuscript.
5. The English can be improved.
Comments on the Quality of English LanguageThe English can be improved.
Author Response
We thank Reviewer 1 for the through reading of the manuscript and for Reviewer’s remarks. The remarks helped us improve quality and readability of our manuscript. We agree with most of remarks and we made appropriate corrections to the manuscript. Highlighting changes is not possible because nearly all sentences were rephrased, meeting the remark about incorrect English. Figures were also edited. Duplicate references were removed.
Remark 1. What are the reasons behind the very low rotating speed (10 rpm, 50 rpm) RDE experiments?
Response. May be, we do not understand the remark. If the Reviewer meant that vibrations from the desk, laboratory floor, etc. interfere with the electrolyte flow caused by the electrode rotation, the answer is the following. According to Fig. 2, current density values at plateau (ipl) fall within the same trend ipl –ω0.5. Thus, the possible vibrations do not cause significant perturbations at these RDE rotation rates. Furthermore, in the case of substantial perturbations of the electrolyte flow, the measured current density is expected to exceed the current density calculated by Levich equation.
Remark 2. The deviation from Levich’s dependence is so large from 11% to 40%. Why? Do you try to fit the data with Koutecký–Levich dependence (ω-0.5-Ilim) considering the Koutecký–Levich dependence was first derived for a chemical-electrochemical reaction (Levich, V. G. Physicochemical Hydrodynamics; Prentice-Hall, Inc., 1962; pp 345−357)?
Response. We agree with the Reviewer. In Levich, V. G. Physicochemical Hydrodynamics; Prentice-Hall, Inc., 1962; pp 345−357) and in the article [J. Koutecky and V.G Levich, Application of rotating disk electrode to studies of kinetic and catalytic processes in electrochemistry, Zh. Phys. Chem 1958, 32, 7, 1565-1575 (Russian)] the following reaction sequence was considered:
nA↔mB, B+ ne →C. There is no autocatalysis in this reaction sequence. So, this is not our case.
The following autocatalytic reaction sequence was mentioned in these works, but it was not solved:
nA + mB → C; C + ne → A.
Much later, the theory of autocatalytic process on RDE was elaborated by M.A. Vorotyntsev and coworkers ref. [25]. In our work we actually compare our results with Levich equation for mass-transport limited currents at RDE.
As concerns Levich a equation, in our work we use widely adopted expression (12) e.g. [Faulkner, L.R.; Bard, A.J. Electrochemical Methods: Fundamentals and Applications ref. 28.]
In our research, measured plateau current densities are always lower than the values calculated by Levich equation (12). The observed deviations from Levich plot in our work we ascribe to all the autocatalytic chemistry which is not taken into account in pure mass-transport limited kinetics (Levich equation).
Remark 3. You mention that the iodine layer is thin. Could you provide the exact thinness of the layer after the RDE by some instruments, say SEM?
Response. Unfortunately, we do not see a relatively simple way of measuring the iodine layer thickness in a short time given by the Editor. Our RDE cannot be disassembled. It is too high to be placed in a vacuum chamber of available SEM. According to our research, the presence of iodine layer at potentials lower than equilibrium potential of iodide/iodine pair, E<0.65V, is a result of balance between production of iodine on the electrode and its removal from the electrode surface. In stationary process the rates are equal. At stationary electrode the thickness could be different. In experiments with RDE “volume” of electrolyte where prodiced iodide reacts with iodate is far from the electrode. The presence of reaction products, actually iodine, formed in the “volume” can be ignored as long as iodine concentration in the cell is low. The cell “volume” is stirred by RDE rotation, so, iodine concertation outside the diffusion layer is the same. In case of stationary electrode diffusion layer thickness increases with time. Autocatalytic process at stationary electrode requires theoretical modeling. Iodine removal from the stationary electrode is maintained only by diffusion through the diffusion layer. But solubility of iodine is low. If saturation concentration of iodine is lower than concentration of iodic acid in the electrolyte, than stationary process seems impossible. Electrode will be coated by blocking iodine layer. Most probably the layer would be thick and porous. In ref [17 A. Modestov; V. Andreev; A. Antipov. Aluminum/Bromate and Aluminum/Iodate Mechanically Rechargeable Batteries, Batteries 2022, 8(12), 270] we conducted experiments with flow-through carbon felt electrode of H2/HIO3 flow cell. At cell voltage lower than 0.7 V it was really hard to avoid complete blocking by iodine of flow of aqueous 0.4 M KIO3-1 M H2SO4.
Minor points:
- The quality of all figures in the main article can be increased.
Quality of all figures was increased.
- Line 194: The Figure 1. should be Figure 2.
Corrected
- Line 197: Measurements in 0.01M HIO3– 0.5M H2SO4should be Measurements in 0.05 M HIO3 – 1 M H2SO4
Corrected
- Line 194−Line 197: The SPACE is needed between the numerical value and unit symbol. Please also check other parts of this manuscript.
Corrected
- The English can be improved.
Text of the manuscript was edited. No more than 10 sentences in the whole manuscript remain unchanged.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript titled "Autocatalyzed kinetics of 6-electron electroreduction of iodic acid studied by rotating disk electrode technique" presents a detailed study on the electrochemical reduction of iodate to iodide, which is relevant for the development of redox flow batteries. The topic is attractive and falls within the scope of Catalysts. Some revisions are necessary before publication.
1. While the manuscript describes a complex autocatalytic process, it could benefit from a more straightforward or visual schematic diagram of the proposed mechanism to help readers better understand the process.
2. The study relies heavily on the rotating disk electrode technique. It is necessary to include a control experiment or comparison with regular glassy carbon electrodes.
3. Some of the voltammograms are dense and complex. Please simplify or break them down for better readability.
4. The manuscript should consider adding a statistical analysis of the results to provide a rigorous assessment of the data variability and significance.
5. The manuscript could benefit from a more thorough comparison with existing literature, especially studies that have investigated similar systems or mechanisms. This would help position the current work within the context of ongoing research.
6. The manuscript should be reviewed for language and grammar to ensure that it is free from errors that could detract from the scientific content.
Comments on the Quality of English LanguageExtensive editing of English language required.
Author Response
We thank the Reviewer 2 for through reading of the manuscript and for Reviewer’s remarks. The remarks helped us helped improve quality and readability of our manuscript. We agree with most of remarks and we made appropriate corrections to the manuscript. Highlighting changes is not possible because nearly all sentences were rephrased, meeting the remark about incorrect English. Figures were also edited. Duplicate references were removed.
Remark 1. While the manuscript describes a complex autocatalytic process, it could benefit from a more straightforward or visual schematic diagram of the proposed mechanism to help readers better understand the process.
Response. Simplified scheme of HIO3 electrochemical reduction to iodide in acidic electrolytes at GC electrode is added as Figure 5. The scheme underlines important role of the iodine layer in the autocatalytic cycle.
Remark 2. The study relies heavily on the rotating disk electrode technique. It is necessary to include a control experiment or comparison with regular glassy carbon electrodes.
Response.
According to our research, the presence of iodine layer at potentials lower than equilibrium potential of iodide/iodine pair, E<0.65V, is a result of balance between production of iodine on the electrode and its removal from the electrode surface. In stationary process the rates are equal. At stationary electrode the thickness could be different. In experiments with RDE “volume” of electrolyte where iodide reacts with iodate is far from the electrode. The presence of reaction products, actually iodine, formed in the “volume” can be ignored as long as iodine concentration in the cell is low. The cell “volume” is stirred by RDE rotation, so, iodine concertation in the electrolyte outside the diffusion layer is the same. In case of stationary electrode diffusion layer thickness increases with time. Autocatalytic process at stationary electrode requires theoretical modeling. Iodine removal from the stationary electrode is maintained only by diffusion through the diffusion layer. But solubility of iodine is low. If saturation concentration of iodine is lower than concentration of iodic acid in the electrolyte, than stationary process seems impossible. Electrode will be coated by blocking iodine layer. In ref [17 A. Modestov; V. Andreev; A. Antipov. Aluminum/Bromate and Aluminum/Iodate Mechanically Rechargeable Batteries, Batteries 2022, 8(12), 270] we conducted experiments with flow-through carbon felt electrode of H2/HIO3 flow cell. At cell voltage lower than 0.7 V it was really hard to avoid complete blocking by iodine of flow of aqueous 0.4 M KIO3-1 M H2SO4. In the experiments with flow cell H2/HIO3-H2SO4 the cycle of transformations from HIO3 to iodide and back was conducted.
Remark 3. Some of the voltammograms are dense and complex. Please simplify or break them down for better readability.
Response. Quality of all figures was increased. We ask the Reviewer to indicate what figures are to be improved.
Remark 4. The manuscript should consider adding a statistical analysis of the results to provide a rigorous assessment of the data variability and significance.
Response. At present stage we have no mathematical model of HIO3 reduction in acidic electrolytes. For this reason, we cannot verify the model by experiments and provide statistical data showing relevance/irrelevance of the model. The primary goal of the manuscript is to show that iodates can be reduced at high rates at carbonaceous electrodes. Indeed, measured HIO3 reductions currents are lower than values of convective diffusion limited currents calculated by Levich equation. But it is hard to exceed the diffusion rate in real chemical/electrochemical process. The difference between experimental values of current density and values calculated by Levich equation reaches 40%. It is ascribed to relatively slow autocatalytic reactions. The difference is shown in Figure 2. The by-product of the study is a hypothesis of existence of iodine layer on the electrode during HIO3 reduction at potentials lower than equilibrium potential of iodine/iodide redox pair.
Remark 5. The manuscript could benefit from a more thorough comparison with existing literature, especially studies that have investigated similar systems or mechanisms. This would help position the current work within the context of ongoing research.
Remark 6. The manuscript should be reviewed for language and grammar to ensure that it is free from errors that could detract from the scientific content.
Response.
Text of the manuscript was edited. It seems that no more than 10 sentences in the whole manuscript remain unchanged.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThis reviewer thanks the authors for addressing the comments raised and agrees to publish the manuscript.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors addressed all the concerns of the reviewer. The paper can be published in its current form.
