Irreversibility of Ag|AgCl Reference Electrodes

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript addresses a fundamental but important issue in the interfacial reaction kinetics of Ag|AgCl reference electrodes. Using AC impedance spectroscopy, the authors systematically characterize the charge transfer behavior of AgCl films with different deposition loadings, determine the standard charge transfer rate constant on the order of 10⁻⁹ cm s⁻¹, propose an empirical power-law relation between exchange current density and AgCl surface loading (with an exponent of 0.3). The topic addresses practical demands in miniaturized electrochemical systems and provides useful guidance for the rational design and application of reference electrodes. The manuscript is logically structured with substantial experimental data. I recommend the acceptance of this manuscript after some minor revisions as follows:

1. The authors use three different pH solutions (2.9, 7.0, 8.4) but do not systematically investigate or discuss the effect of pH on AgCl formation or kinetics.
2. The surface morphology of the cylindrical silver wire may lead to non-uniform AgCl deposition, which directly affects the accuracy of the calculated surface loading. Please supplement a description of how the thickness uniformity of the AgCl film was ensured during the deposition process.
3. The authors reported j₀ values several orders of magnitude lower than those in Table 1. The authors simply attribute the discrepancy to "fractal area much larger than geometrical area". A more critical discussion of why previous studies overestimated j₀ (e.g., due to uncompensated resistance, film porosity, or misinterpretation of EIS) is needed.
4. The manuscript cites several older references but omits recent studies on Ag|AgCl thin-film electrodes, especially those using advanced EIS modeling or in situ characterization. Including such references would strengthen the context.
5. Some equations are misnumbered. For example, Eq. (7) appears twice.
6. References have multiple formatting irregularities. For example, Ref.12 lacks DOI and journal name abbreviations, volume and page numbering are inconsistent. Please unify the reference format according to the journal's author guidelines and supplement missing information.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

 

Irreversibility of Ag|AgCl reference electrodes

The authors described the importance of using AC-impedance to estimate electron transfer kinetics as opposed to use cyclic voltammetry which adds strength to this study. Surface confined reactions coupled with electron transfers are challenging to study, and the authors show that using AC-impedance is a viable option to estimate reaction rates using the charge transfer resistance (Rct). Overall, the study shows that Ag/AgCl is a reaction that is not reversible when larger currents are being measured. This induces large deviations when measuring electrochemical potentials and how this deviation is related to AgCl thickness. The authors suggest that using this chemical reaction as reference electrode can be used when electrochemical setups with microelectrodes are employed as the current range is lower in the nA.

Overall, although the study is interesting, it lacks experiments that will add value to this manuscript. An important experiment the authors should consider is to demonstrate that when microelectrodes are used in a two-electrode setup this deviation is not significant.

 

Reviewer comments

 

1. In the first paragraph of the results and discussion, the authors explained the shape of the voltammetry, however, the shape is very well-known for deposition of metals and it has been described in electrochemical text books, in general electron transfer reactions coupled with adsorption steps, produce this type of voltammetry, where there is no diffusional limited current, the authors should consider adding a reference to this statement.

 

2. Figure 1b shows the voltammetry at narrow potential window, from -0.1 V to 0.1 V, the authors claim there is no scan rate dependence, but this potential range is not sufficient evidence to support this claim. There was no justification for why this narrow potential window experiment was performed. It is recommended that the authors consider adding the full potential window at different scan rates to support the statement of “Therefore, the currents should be controlled with chemical reaction rates rather than mass transport”.

 

 

3. The authors on page 4 at the beginning claim the process is reversible due the fact of lack of scan rate dependency, however, this statement should be revisit, reversibility on an electrochemical reaction, specifically looking at the voltametric peaks is describe as the separation between the anodic and cathodic being 59 mV/n for a purely diffusional process, but for a process where adsorption and deposition of metals occurs should be 0 V. This is described in the textbook of “Electrochemical methods: Fundamentals and Applications” Chapter 13.

 

4. On page 7 of this manuscript, the authors explained in the last paragraph how the electrochemical potential deviates significantly when a two-electrode setup is used. The authors suggest that using microelectrodes can reduce this deviation because the currents are in the range of nA. It is recommended that the authors consider adding evidence to support this statement.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors study kinetics of the Ag|AgCl redox couple depend on thickness using impedance spectroscopy to determine electron transfer rate constants. The result is significant for improving understanding of the conditions under which a Ag|AgCl electrode can be a counterelectrode in ultramicroelectrode electrochemistry. The result specifies a maximum current density above which Ag|AgCl would not be a good counterelectrode. There are some suggestions for revision/improvement as follows:

(1) the result of equation (1) should be called as a 'surface density' given the units

(2) specify the NaCl concentration for Figure 1 experiments in the caption.

(3) Does the position of the peak in 1A depend on Cl- concentration? Would it peak at a higher potential for a higher concentration? How is it affected by scan rate?

(4) What is the meaning of the slope of 7 in Figure 2A, is it related to the CPE exponent? Discuss in more detail.

(5) What are the Rct values from Figure 3?

(6) Explain how the exponent for the trend results in the conclusion that the surface is 'fluctuated'. What distinguishes particles that are part of the surface but not electroactive?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

All my comments have been addressed. 

Thanks to the authors.