Round 1

Reviewer 1 Report

The authors reported their work on titled, Use of inner/outer sphere terminology in electrochemistry - a hexacyanoferrate II/III case study. Although, this work contains some results, the organization and interpretation of the result should be enhanced further. Hence, I recommend this work required a substantial revision before considering for publications.

1.     Provide the obtained results in the abstract in more concise.

2.     Novelty of the work should be highlighted in the introduction in more clearly.

3.     Synthesis process should be improved with more explanation.

4.     The authors designed the work nicely, merely presented the results but failed to discuss the observed results elaborately.

5.     I suggest the authors to compare the previous literature similar to that work to find a merits of this work.

6.     Refer and include the following references to strengthen the current version of the draft; Renewable & Sustainable Energy Reviews, 143 (2021) 110849; Applied Catalysis B: Environmental 316 (2022) 121603.

The authors reported their work on titled, Use of inner/outer sphere terminology in electrochemistry - a hexacyanoferrate II/III case study. Although, this work contains some results, the organization and interpretation of the result should be enhanced further. Hence, I recommend this work required a substantial revision before considering for publications.

1.     Provide the obtained results in the abstract in more concise.

2.     Novelty of the work should be highlighted in the introduction in more clearly.

3.     Synthesis process should be improved with more explanation.

4.     The authors designed the work nicely, merely presented the results but failed to discuss the observed results elaborately.

5.     I suggest the authors to compare the previous literature similar to that work to find a merits of this work.

6.     Refer and include the following references to strengthen the current version of the draft; Renewable & Sustainable Energy Reviews, 143 (2021) 110849; Applied Catalysis B: Environmental 316 (2022) 121603.

Author Response

Changes in the final text of the submitted manuscript in response to the reviewers’ concerns are highlighted in yellow.

Reviewer 1.

The authors reported their work on titled  Use of inner/outer sphere terminology in electrochemistry - a hexacyanoferrate II/III case study. Although, this work contains some results, the organization and interpretation of the result should be enhanced further. Hence, I recommend this work required a substantial revision before considering for publications.

This is a good suggestion. The Discussion section of this review article has now been now broken down into distinctive sections more clearly highlighting a diverse range of effects which appear to influence the electron transfer behaviour of the hexacyanoferrate II/III species in solution. Comparisons are made taking into account such effects as the presence or absence of oxygen, influence of cations, occurrence of surface adsorption, use of various different carbon and metallic electrodes, double layer effects and hydrophilicity/hydrophobic effects amongst many others.  The various sections are now more clearly delineated in the text and are numbered accordingly.

1. Provide the obtained results in the abstract in more concise.

As it is a review paper it does not describe experimental results in great detail. Instead it demonstrates the confusion caused by use of different labels for hexacyanoferrate II/III electron transfer processes (inner sphere cf. outer sphere). The Abstract initially highlights the importance of the hexacyanoferrate II/III anions which often serve as redox probe species before highlighting different situations where it seems to behave in a unpredictable manner. Thus ascribing it to one or the other process without taking such effects into account is problematic. Suggestions to address this quandry are clearly described both in the Abstract itself and in the subsequent text. The Abstract has been shortened slightly, with the final sentence removed.

2. Novelty of the work should be highlighted in the introduction in more clearly.

This is a good suggestion and additional sentences have now been added in the Introduction (highlighted in yellow) making this more readily apparent. The point is made that although some experienced electrochemists may be aware of the problem of mis-labelling the hexacyanoferrate II/III electron transfer process, many inexperienced electrochemists will not understand the distinction. The novelty of this article lies in highlighting this issue through a systematic review of the effects of a wide range of factors that can influence the electron transfer processes in the hexactanoferrate II/III case. These can make attribution to an inner sphere or an outer sphere electron transfer process problematic. Ways to overcome this are suggested, either through use of the terms multi sphere or surface sensitive.

3. Synthesis process should be improved with more explanation.

This comment is a bit perplexing as the work does does not deal with chemical or electrochemical synthesis. In terms of the submitted paper’s structure, following the Introduction and a short section on the molecular and electrochemical characteristics of the hexacyanoferrate II/III species, the Discussion section identifies a number of key issues which may influence the electron transfer behaviour of the hexacyanoferrate II/III anions. These  factors are elaborated upon followed by a Conclusion section with future work plans and recommendations.

4. The authors designed the work nicely, merely presented the results but failed to discuss the observed results elaborately.

This seems a bit contradictory. The Discussion section goes into considerable depth and with over 150 references there is reference to a major body of literature dealing with the hexacyanoferrate II/III redox system, much of it published in recent years.

5. I suggest the authors to compare the previous literature similar to that work to find a merits of this work.

An extensive examination of the literature was described in the submitted article, including those of highly cited authors such as Banks (reference 10) and McCreery (references 13 and 43)  amongst others, who have examined a number of such redox probe species in considerable detail. Their approaches and categorisation are clearly described in the submitted paper.

6. Refer and include the following references to strengthen the current version of the draft; 

The authors are very grateful for the suggestion to review these two recently published papers with a view to adding them to the submitted article.

(i)                  S Jun Lee, J Theerthagiri, P Nithyadharseni, P Arunachalam, D Balaji, A M Kumar, J Madhavan, V Mittal, M Y Choi,  Heteroatom-doped graphene-based materials for sustainable energy applications: A review; Renewable & Sustainable Energy Reviews, 143 (2021) 110849; 

 

This most interesting review article deals with the use of doped graphene materials serving as electrodes in electron transfer processes which are of particular importance in a variety of energy conversion and storage technologies. These include batteries, dye-sensitised solar cells, supercapacitors as well hydrogen production from electrocatalytic water splitting. A sentence (highlighted in yellow) citing this useful article has now been incorporated in Section 3.5

and the reference has been added to the list of references (as reference 156)..

(ii) Y Yu , S Jun Lee, J Theerthagiri , Y Lee , M Y Choi, Architecting the AuPt alloys for hydrazine oxidation as an anolyte in fuel cell: Comparative analysis of hydrazine splitting and water splitting for energy-saving H₂ generation Applied Catalysis B: Environmental 316 (2022) 121603.

This second paper describes an interesting study which investigates the feasibility of the use of corrosion-resistant AuPt alloys for stimulating hydrazine N₂H₄ fuel cells. Here the sluggish anodic oxygen evolution reaction (OER) is replaced with the hydrazine (N₂H₄) oxidation reaction (HzOR) which is suggested to be a more effective approach for achieving energy-saving hydrogen (H₂) fuel production in water electrolyzers. Replacement of oxygen in such systems could achieve both enhanced electrical energy generation and higher rates of H₂ fuel production.  Careful inspection of this article however reveals no reference to any of the key topics dealt with in the submitted Electrochem paper, such as the terms inner sphere, outer sphere, multi sphere, ferrocyanide, ferricyanide, hexacyanoferrate, surface sensitive, surface insensitive. This makes it difficult to justify its incluson as an additional reference. 

Reviewer 2 Report

A pleasure to read.

Although I am inclined to suggest that any attempt to label a given electrode reaction as ISET or OSET is void of any practical value and should be discouraged, I am also aware that it will not happen. Therefore a review specifically focusing on this is very important and timely.

The last author name is missing from the author list (nothing after "and").

 

Author Response

Reviewer 2

A pleasure to read.

The authors are most appreciative of this comment.

Although I am inclined to suggest that any attempt to label a given electrode reaction as ISET or OSET is void of any practical value and should be discouraged, I am also aware that it will not happen. Therefore a review specifically focusing on this is very important and timely.

The authors agree and once again they are most appreciative of this supportive comment.

The last author name is missing from the author list (nothing after "and").

This formatting problem has now been corrected. In the article submitted the word “and” was missing between the last two authors’ names. It has now been inserted between their names. There are only three authors.

Reviewer 3 Report

In this review article, the authors call into question the use of “inner/outer sphere electron transfer” (ISET/OSET) terminology focusing specifically on salts of hexacyanoferrate II/III. Their discussions are based on a solid and comprehensive literature database and their call into question of the ISET/OSET terminology is properly justified. Thus, the authors conclude that the heterogeneous electrochemical surface reactions involving the hexacyanoferrate II/III redox species may utilise either ISET or OSET processes, depending on the conditions employed in the electrochemical study being undertaken, creating thus confusion especially for young electrochemists. The authors carefully analyse the current literature, they highlight and discuss the reasons for such classification disparity and even suggest an alternative classification system for utilising hexacyanoferrate II/III redox species.

The authors did a very nice and detailed work, which could be even considered for educational purposes, however the authors need to pay more attention to the layout of the review, numbering of chapters and use of headings and references in accordance with the journal template. Some minor comments (since the page numbering is completely chaotic, the reviewer will use quotations):

·       “ions in aerated 0.1 M KCl at 50mV/s in N-doped amorphous carbon (left) and (right) in Highly Oriented Pyrolytic Graphite (HOPG) showing a range of ΔEp values [30].” – whole paragraph seems out of place

·       Table 1 – the reviewer suggests to move the reference column as last column

·       “…yielding reasonably close HET ko values between 1.6 x 10-3 to 3.1 x 10-3 cms-1 in deaerated 0.1 N Na2SO4” – it should be 0.1 M Na2SO4

·       “….including (a) double layer effects (14.7) (b) the effect of the metal on structure on the IHP (hydrophobicity) and (c) the effect of the energy and distribution of electronic states (DOS) in the electrode (3.5.5)”- what are those numbers in parentheses?

·       Did the authors get permission to use Fig 2 and 3 from the appropriate source? Besides this, Fig 2 has no citation.

Author Response

Reviewer 3.

In this review article, the authors call into question the use of “inner/outer sphere electron transfer” (ISET/OSET) terminology focusing specifically on salts of hexacyanoferrate II/III. Their discussions are based on a solid and comprehensive literature database and their call into question of the ISET/OSET terminology is properly justified. Thus, the authors conclude that the heterogeneous electrochemical surface reactions involving the hexacyanoferrate II/III redox species may utilise either ISET or OSET processes, depending on the conditions employed in the electrochemical study being undertaken, creating thus confusion especially for young electrochemists. The authors carefully analyse the current literature, they highlight and discuss the reasons for such classification disparity and even suggest an alternative classification system for utilising hexacyanoferrate II/III redox species.

The authors are most grateful for the reviewer’s insightful comments. They obviously spent a considerable amount of time conducting their review of the paper. As it is quite a long review article the authors are especially appreciative.

The authors did a very nice and detailed work, which could be even considered for educational purposes, however the authors need to pay more attention to the layout of the review, numbering of chapters and use of headings and references in accordance with the journal template.

The authors agree with this and consequently significant changes have been made to the final manuscript. Book chapters, where appropriate have been added to the books listed in the references at the end of the paper. These changes are highlighted in yellow in the revised manuscript.

Some minor comments (since the page numbering is completely chaotic, the reviewer will use quotations):

The page numbering issue has now been rectified in the re-submitted manuscript.

• “ions in aerated 0.1 M KCl at 50mV/s in N-doped amorphous carbon (left) and (right) in Highly Oriented Pyrolytic Graphite (HOPG) showing a range of ΔEp values [30].” – whole paragraph seems out of place

       This text was originally part of the legend for Figure 2 and has now been incorporated back into this labelling of the figure..

• Table 1 – the reviewer suggests to move the reference column as last column

This is a very good suggestion and it has now been done.

• “…yielding reasonably close HET ko values between 1.6 x 10-3 to 3.1 x 10-3 cms-1 in deaerated 0.1 N Na2SO4” – it should be 0.1 M Na2SO4

This refers to results obtained from reference 79 (by Michael Noel and P. N. Anantharaman, 1985) who use the older concentration convention, namely normality (N). In fact 0.1 N Na₂SO₄ is equivalent to 0.05 M Na₂SO₄ or 0.05 mol/L  Na₂SO₄. This is now noted in the text (and highlighted in yellow) in section 3.3.

• “….including (a) double layer effects (14.7) (b) the effect of the metal on structure on the IHP (hydrophobicity) and (c) the effect of the energy and distribution of electronic states (DOS) in the electrode (3.5.5)”- what are those numbers in parentheses?

These numbers are superfluous and have now been removed from the revised manuscipt.

• Did the authors get permission to use Fig 2 and 3 from the appropriate source?

These figures are both taken from a paper by Hamal et al published by MDPI (reference 30).

Hamal K;  May J; Zhu H; Dalbec F; Echeverria E; McIlroy D ;, Aston ; Cheng I F , Electrochemical Aspects of a Nitrogen-Doped Pseudo-Graphitic Carbon Material: Resistance to Electrode Fouling by Air-Aging and Dopamine Electro-Oxidation, J Carbon Res. 2020, 6(4), 68;   doi.org/10.3390/c6040068

These are the same publishers of the journal to which this article was submitted. Both journals are Open Access publications. The authors were not sure how to obtain permission from the journal from which the figure was taken or even if this was necessary in this case.

Besides this, Fig 2 has no citation.

The citation is now placed at the end of the figure legend which was broken up and separated from the earlier part in the paper for review. It has now been put back together again and the reference [30] is now placed at the end of the completed figure legend.

 

 

 

Reviewer 4 Report

The review presented is a very important piece that fills a gap in the electrochemical concepts of outer and inner sphere redox probes in the use that these concepts have been made in the literature with the ferrocyanide probe. The review contains an extensive bibliography, pertinently chosen and effectively described focusing on the problem at hand. All this makes the work very appropriate to be published in the journal Electrochem. However, a series of corrections are recommended to make the work more comprehensible by the potential reader.

First, there is some disconnect in the numbers of literature citations. Since the authors have chosen the numbering, the references must appear in the text correlatively. In several sections of the work, jumps are observed in the numbering, with faults that are followed in later sections. Same can be said for the table´s references. Also, the page numbers change suddenly when the table is introduced¡ Also, the way of presenting them in the text changes throughout the sections. However, it seems that the citations are appropriate and located in the proper places.

On the other hand, the titles of the sections (the introduction is numbered (1), but then there are some epigraphs that look like section headings, but it is not clear since neither the font nor the indentation changes... Sometimes it is detected also that there is a paragraph change that really is not, so it is difficult to unite the ideas. The equation numbers must be revised and their correspondences in the text.

Another suggestion to the authors is to simplify the comments made on the references, since repetition of data and conditions are sometimes observed.

In general, an exhaustive revision of the text is recommended to make it more readable. Moreover, an index with the structure of the work as well as a glossary of terms, should be well acknowledged.

Other details as follows.

In page 3, the comment: “An example is the determination of the electrochemically active (electro-active) surface area of an electrode, (ECSA or EASA), which requires fast electron transfer (ie a reversible process) and a redox system limited purely by diffusion control. [10, 11, 12] For a simple one electron transfer process such as that involving hexacyanoferrate II/III anions commonly used approaches, include cyclic voltammetry (in conjunction with the

Randles-Ševčík equation) and chronoamperometry (utilising the Cottrell equation). These can be adapted, to determine the EASA, provided the diffusion coefficients of the reactants are accurately known. [3, 44].”.

Although the processes analyzed under these conditions are diffusion controlled and useful to determine the geometric area of the electrode, the electrochemically active surface area must take into account the microscopic features of the surface. Although it is difficult to have the true surface area for graphitic materials, and this is an easy method to have the real area, it must be noted that the accuracy of these value is strongly dependent on the roughness of the material.

 

Page 4, 6^th line from bottom: 0.3v^1/2: can the authors explain more deeply the meaning of this factor?

Author Response

please see the attachment

 

Author Response File: Author Response.pdf