Ni-Co Electrodeposition Improvement Using Phenylsalicylimine Derivatives as Additives in Ethaline-Based Deep Eutectic Solvents (DES)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study proposes a new method of using phenylsulfonimide (PSI) derivatives as electroplating additives for nickel-cobalt alloys by using ethanol as a deep eutectic solvent (DES). The effects of changing functional groups on charge and mass transfer were investigated. The results showed excellent performance and compliance with relevant standards. Therefore, this paper has certain guiding significance and is worth recommending for publication. However, there are some deficiencies that need to be improved.
1. The title of this paper is not closely related to the main content and needs to be rewritten.
2. The structure of the abstract does not meet the requirements. The word count is too short, and it does not introduce the research background. Moreover, there are no quantifiable research data, and the significance of this study should be added at the end of the abstract. Therefore, it needs to be rewritten.
3. In the introduction, only a simple list of related literature is provided without pointing out the existing problems and shortcomings, which fails to highlight the innovation of this paper. Therefore, a more in-depth discussion of the existing literature is needed.
4. There are two Figure 1s in the text. Please check and modify.
5. The text duplication rate of this paper is relatively high, reaching 35%. It is necessary to reduce the text duplication rate.
6. The four sub-figures in Figure 3 need standard numbering and cannot be introduced by left and right. Please modify. The same problem exists in Figures 2 and 6.
7. Figure 5 in the text is not standardized. Each sub-figure needs to be numbered and explained in the title. Please modify.
8. Please explain how changing functional groups can affect charge and mass transfer.
9. The conclusion part of this paper is not well-written and does not highlight the innovation of this paper. It feels like a conclusion of an experimental report. It is recommended that the author rewrite it and describe it in bullet points.

Author Response

General answer

We sincerely appreciate the comments and opinions that we have considered to improve the quality of our research. To provide a more detailed response to the comments, they have been divided as follows

1. The title of this paper is not closely related to the main content and needs to be rewritten.

We agree, the title was changed to improve the relationship with the content,  as shown below:

“Ni-Co electrodeposition improvement using phenylsalicylimines derivatives as additives in Ethaline-based deep eutectic solvents (DES)”
2. The structure of the abstract does not meet the requirements. The word count is too short, and it does not introduce the research background. Moreover, there are no quantifiable research data, and the significance of this study should be added at the end of the abstract. Therefore, it needs to be rewritten.

Answer C2: The abstract was improved as shown below.

The development of metallic coatings, such as Ni-Co alloys, with a particular emphasis on their homogeneity, processability, and sustainability, is of utmost significance. To address these challenges, the utilization of phenylsalicylimines (PSI) as additives within deep eutectic solvents (DES) was investigated, assessing their influence on the electrodeposition process of these metals at an intermediate temperature of 60°C, while circumventing aqueous reaction conditions. The findings demonstrated that the incorporation of PSI markedly enhances coating uniformity, resulting in an optimal cobalt content of 37% and an average thickness of 24 µm. Electrochemical evaluations revealed improvements in charge and mass transfer, thereby optimizing process efficiency. Moreover, computational studies confirmed that PSI forms stable complexes with Co(II), modulating the electrochemical characteristics of the system through the introduction of the diethylamino electron-donating group, which significantly stabilizes the coordinated forms with both components of the DES. Additionally, the coatings displayed exceptional corrosion resistance, with a rate of 0.781 µm per year, and achieved an optimal hardness of 38 N HRC, conforming to ASTM B994 standards. This research contributes to the development of electroplating bath designs for metallic coating deposition and lays the groundwork for the advancement of sophisticated technologies in functional coatings that augment corrosion resistance and mechanical properties.

3. Therefore, a more in-depth discussion of the existing literature is needed.

The introduction was revised to enhance detail, incorporating valuable feedback as shown below:

Nickel and cobalt superalloys are prized in the aerospace industry for their remarkable strength and stability at high temperatures, making them perfect for turbine engines and jet propulsion systems. These superalloys are capable of withstanding extreme temperatures and resisting stress and corrosion [1,2]. Electroplating is crucial for manufacturing these alloys because it enables precise control over their composition and structure, thereby enhancing their overall performance and properties. The alloy formation process presents numerous challenges and key factors that can impact quality and efficiency. Common issues involve internal stresses within the coating caused by hydrogen release, as well as the need for precise control over the microstructure and chemical composition of nickel-cobalt superalloy coatings, which significantly influence their mechanical properties, such as hardness and fatigue resistance. A pivotal aspect in the mechanism of alloy formation is the rapid nucleation of cobalt in comparison to nickel, as this constitutes an anomalous codeposition type. [3,4]. To achieve controlled thickness, uniform coatings, and defect-free surfaces, several alternatives are available for managing this deposition process, including adjustments in concentration, surfactant additives, or complexes that influence ion migration and redox reactions. [5]. The utilization of complexes is constrained by their solubility in aqueous solution and temperature, thereby necessitating the exploration of methods in non-aqueous systems [2,6].

The development of environmentally friendly and cost-effective electroplating processes that meet aerospace standards introduces an additional layer of complexity. Deep eutectic solvents (DES) represent a novel, eco-friendly approach that offers an alternative to conventional electrolytes in Nickel, cobalt, and chromium processes. This approach has the potential to mitigate carcinogenic risks, reduce water and energy consumption, and facilitate the evaluation of a broad spectrum of organic compounds soluble in ethaline [7-9].

Recent studies have centered on the role of binders as additives in metal electroplating. These additives have been shown to have a significant impact on ion migration, concentration polarization, reduction potential, and electrochemical crystallization of electrodeposited grains. These elements have been demonstrated to enhance the microstructure and faradaic performance of deposits [10-13]. Alesary H.F. et al. showed that the signals in cyclic voltammetry changed with different amounts of nicotinic acid in DES, altering the structure parallel or perpendicular to the electrode [14]. Andrew P. Abbott's group investigated the role of additives in nickel and cobalt electroplating processes. Their findings revealed that nicotinic acid, ethylenediamine, and boric acid exert a limited effect on the amount of nickel deposited. However, these additives have been observed to influence ionic reduction by forming Ni(II) complexes, thereby affecting the electroplating process. This, in turn, alters the diffusion rate and concentration of metal ions [15,16].

The selection of appropriate ligands necessitates the consideration of the structure of complexes with the metals to be deposited. For instance, nickel (II) complexes exhibit octahedral geometry, while cobalt (II) complexes manifest tetragonal geometry. These orbitals enable the rapid formation of coordination complexes with Lewis bases through electron donation [17-22]. Schiff bases are classified as azomethines or imines with organic side chains, and they are represented by the general formula R1R2 C=NR3. The synthesis of these compounds is straightforward, enabling the formation of stable chelates with metal ions through the azomethine, hydroxyl, and amine groups. This facilitates charge and mass transfer during the electroplating process of metal alloys. The Schiff base H2L and its metal chelates have been synthesized and characterized as binuclear complexes using polar organic solvents [23-27].

Phenylsalicylimines (PSI) are easily synthesized compounds that can be formed using inexpensive raw materials and can form metal cation complexes through chelated rings, making them potential candidates for selective bonding with cobalt and nickel [28-30]. These ions have been found to form stoichiometric complexes with Co(II), Ni(II) and Cu(II) in the form [M(L)X]X and [M(L)SO4]. L is 3,3'-thiodipropionic acid bis(4-amino-5-ethylimino-2,3-dimethyl-1-phenyl-3-pyrazoline)[31-36]. Additionally, they are low-cost compounds, which favor their implementation in industry; they are a favorable alternative for sustainable use in Ni-Co electroplating.

Most existing research has focused on enhancing nickel mobility, primarily to cobalt ions. This study explores two Phenylsalicylimines compounds with modified functional groups to hinder cobalt ion mobility during nickel-cobalt alloy deposition in ethaline, a Deep Eutectic Solvent.
4. There are two Figure 1s in the text. Please check and modify.
The number of figures has been corrected.

5. The text duplication rate of this paper is relatively high, reaching 35%. It is necessary to reduce the text duplication rate.
   The document was analyzed in Compilation. Here is the link to the report if you would like to consult it(https://drive.google.com/file/d/1WoEEx82bJTaCmlzRwtLf9SzE8GVmDBgx/view?usp=drive_link).

However, taking this comment into account, the review was repeated, and areas that may have been detected as duplicates were modified.

6. The four sub-figures in Figure 3 need standard numbering and cannot be introduced by left and right. Please modify. The same problem exists in Figures 2 and 6.
   All images were normalized to meet the journal's quality standards.
7. Figure 5 in the text is not standardized. Each sub-figure needs to be numbered and explained in the title. Please modify.

All images were standardized and numbered to meet the journal's quality standards.

8. Please explain how changing functional groups can affect charge and mass transfer.

Thank you for your valuable comment, which enriches our research. A more detailed explanation of the functional group is included below. (Page 7 line: 250-258)

Changes in the functional groups of phenylsalicylimines alter the charge and mass transfer mechanisms, modifying the reduction rate of nickel and cobalt [52]. It is presumed that the amino group increases the hydrogen bond network and improves the selective complexation capacity of cobalt in ethaline base DES, which is reflected in the solvation and reduction of metal ion transport to the interfacial layer [53-55]. In contrast, the ethylamine functional group leads to the formation of a secondary amine that can coordinate with cobalt and nickel ions, while also influencing the overall charge distribution. As a result, this affects charge migration, transport, and transfer processes, but exhibits a greater response in cobalt oxidation as the current density decreases compared to nickel [56-58]. This preliminary analysis of the interaction between Phenylsalicylimines and ions was further examined through computational calculations, as elucidated below.
9. The conclusion part of this paper is not well-written and does not highlight the innovation of this paper. It feels like the conclusion of an experimental report. It is recommended that the author rewrite it and describe it in bullet points.

The conclusions were improved by taking the comment into account, as shown below.

• Electrochemical analysis demonstrated that the Phenylsalicylimine derivatives exert an effect that inhibits Co(II) migration while augmenting Ni(II) mobility, thus facilitating a more uniform and controlled deposition process. Modifications to the functional group influence redox behavior; notably, the ethylamine group exhibits the most pronounced capacity to reduce Co(II) migration and enhance Ni(II) mobility within Ethaline and DES systems.
• Molecular calculations reveal that the phenylsalicymines ligands with a di-ethylamino functional group (PSI-2) perform two primary roles. It forms stable species with a geometry increasing cobalt's electrostatic attraction to the cathode and improves its mobility by altering solvation and dipole moment. By coordinating with Co(II), it transfers charge, creating positively charged sites that facilitate electrode reactions. The increased dipole moment of Co(II) in square-planar complexes enhances its diffusion to the cathode, using positively charged areas as anchors, and facilitates alloy formation at the Ni-Co interface.
• Utilizing the PSI-2 ligand guarantees a consistent NiCo coating containing 37% Co, which possesses superior anti-corrosion properties and demonstrates exceptionally high microhardness levels. This adequately satisfies the mechanical and anticorrosive resistance requirements established by the ASTM B571 standard.

 

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript presents an interesting study on the “Effect of Phenylsalicylimine derivatives on Ni-Co electroplating in ethaline”. The study discusses in detail the role of PSI additives in the electrodeposition of Co-Ni alloys in deep eutectic solvents. The results presented are interesting, however the presentation of the results in the manuscript needs modifications before publishing.  Therefore, this manuscript could be considered for publication in ‘coatings’ after a revision. Please see few comments and suggestions below.

1. The discussion on the cyclic voltammetry study needs improvement. The results presented lacks clarity.
2. The shift in potentials is not much obvious from the CVs shown. Authors should clearly demonstrate where the shift is?
3. The specified current intensities (23.76 mA, 19.60 mA) are not observed on the CVs directly. How are they measured?
4. The phenol oxidation is mentioned, is it visible in the CVs presented? And also, the potential is against NHE here, while all others are against Ag/Ag(I).
5. Clarity needed for figure 3. What is each colour in the plots correspond to? Em Co1th etc. means what?
6. What is PSI-3 mentioned in figure 4 caption?
7. Figure 5 caption need modification, to include details of all the figures.
8. Extent of crystallization is discussed based on the SEM and EDX data. Is it possible to get crystallization information from SEM data?
9. It is not well established in the text that what additional information is gained from the theoretical calculations. It must be clearly described in the introduction and in the results and discussion.

 

Author Response

General answer

We sincerely appreciate the comments and opinions that we have considered to improve the quality of our research. To provide a more detailed response to the comments, they have been divided as follows.

1. The discussion on the cyclic voltammetry study needs improvement. The results presented lacks clarity.

We agree that the analysis was improved, and the Figures were normalized to enhance clarity, as shown below (Page 6, line 222, and Page 7, line 253).

The voltammograms show the occurrence of reduction processes for nickel and cobalt. Both methods are irreversible and involve a single-step transfer of two electrons for each metal ion, suggesting that less energy is required to reduce both ions in Ethaline (Figure S3, supplementary information).

Figures 3A and 3B show the CVs recorded for 0.5 M Co(II) in DES at different concentrations of additives PSI-1 and PSI-2. In the direct CV scan direction, the reduction peak located in the potential range of −0.4 V to −0.8 V is related to the process associated with the reaction Co (II)DES + 2e- → Co(s). Meanwhile, during the anodic sweep, the oxidation peak, located in the potential range of −0.3 V to 0.1 V, is related to the dissolution of the deposited metal (from Co⁰ to Co²⁺). Focusing on the reduction peaks when PSIs’ are added, there are no changes in the potential window; however, the current density decreases, leading to a reduction phenomenon that requires less energy [48]. In oxidation, the effects of the functional group change in the PSIs’ can be observed, since for PSI-1, there is a potential shift from -0.2 V to 0.7 V and an increase in current output, which effectively reduces cobalt oxidation. For PSI-2, there is no apparent change in potential, but the oxidation current decreases. The cathodic current is suppressed, indicating slower nucleation and growth of cobalt due to interactions between cobalt and the ligand. This is the first evidence suggesting that the -Et2N functional group acts as an additive in inhibiting the oxidation of metallic cobalt at the interface, creating sites that enhance interactions with delocalized electron species. PSI-1 and PSI-2 can be adsorbed onto the electrode surface, inhibiting Co deposition and thereby reducing the growth and nucleation of Co, which increases the nucleation rate. In Figures 3C and 3B, the reduction peak is in the potential range of −0.4 V to −0.7 V associated with the reaction Ni (II)DES + 2e- → Ni(s) and the oxidation peak is in a potential range of −0.3 V to −0.6 V. The PSIs' action of Ni(II) increases in the diffusion-controlled phenomenon, widening the window and accompanied by an increase in current, which favors the nucleation and growth processes at the electrode-solution interface. At the oxidation peak, there is an increase in current, which could be assumed to mean that the compound favors the passivation of the deposited nickel. As the scan progressed with a more positive potential, a significant reduction in current was observed, predictably related to the decrease in solvent. [49-51].

Changes in the functional groups of phenylsalicylimines alter the charge and mass transfer mechanisms, modifying the reduction rate of nickel and cobalt [52]. It is presumed that the amino group increases the hydrogen bond network and improves the selective complexation capacity of cobalt in ethaline base DES, which is reflected in the solvation and reduction of metal ion transport to the interfacial layer [53-55]. In contrast, the ethylamine functional group leads to the formation of a secondary amine that can coordinate with cobalt and nickel ions, while also influencing the overall charge distribution. As a result, this affects charge migration, transport, and transfer processes, but exhibits a greater response in cobalt oxidation as the current density decreases compared to nickel [56-58]. This preliminary analysis of the interaction between Phenylsalicylimines and ions was further examined through computational calculations, as elucidated below.

 

2. The shift in potentials is not much obvious from the CVs shown. Authors should clearly demonstrate where the shift is?

The analysis method was revised to clarify these points, which are not immediately evident from the Figures due to the extensive amount of information. As illustrated in the preceding response.  

3. The specified current intensities (23.76 mA, 19.60 mA) are not observed on the CVs directly. How are they measured?

The wording was modified to facilitate the analysis of the results, which were examined qualitatively rather than quantitatively.

4. The phenol oxidation is mentioned, is it visible in the CVs presented? And also, the potential is against NHE here, while all others are against Ag/Ag(I).

This constitutes an editing error, as another compound possessing a phenol group was previously identified. I apologize for this oversight..

5. Clarity needed for figure 3. What is each colour in the plots correspond to? Em Co1th etc. means what?

Figure 3 has been revised from Figure 4 to enhance the clarity of the data presentation, and an explanation of the displayed data has been incorporated, as follows:

Dissolutions were prepared with 0.5, 1, and 1.5 equivalents of ligand relative to the amount of cobalt present in the solution, subjected to constant agitation at room temperature for 1 hour to evaluate the gradual change in the formation of the corresponding coordination compound. The formation of the expected coordination compound was evidenced by the appearance of a third band centered at 416 nm for PSI1. Meanwhile, for PSI2, only a change in the intensity of the dominant absorption band became evident, as, compared to PSI1, the absorption of the free ligand is bathochromically shifted in the range of 400 to 450 nm due to the presence of the Et 2 N- group. Thus, the effect of the band corresponding to the coordination compound is overshadowed by the initial band of the free ligand.

6. What is PSI-3 mentioned in figure 4 caption?

It appears that further editing oversight has occurred. I sincerely apologize for this mistake.

7. Figure 5 caption need modification, to include details of all the figures.

The caption was amended to specify the information as outlined below.

Figure 5. Potentiodynamic polarization of (A) Co and Ni solutions with 0.03 mM PSI-1 and (B) Co and Ni solutions with 0.03 mM PSI-2 at 333K and 25 rpm.

8. Extent of crystallization is discussed based on the SEM and EDX data. Is it possible to get crystallization information from SEM data?

No, it is impossible; the issue pertains to terminology. It has already been modified to prevent misinterpretation, as demonstrated below:(Page 10, line 336)

The PSI-1 coating (Figure 6B) exhibits a generalized grain morphology and surface segregation, attributed to the increased cobalt content in the deposited alloy compared to the coating without additives. The PSI-2 coating (Figure 6C) exhibits a homogeneous morphology, characterized by homogeneous coating with minimal contrast changes.

9. It is not well established in the text that additional information is gained from the theoretical calculations. It must be clearly described in the introduction and in the results and discussion.

The discussion encompassed the necessity of theoretical calculations, the integration of results into the analysis, and the refinement of conclusions to enhance their relevance to the article. As demonstrated below:

Abstract : (Page 1, line 14 to 31)

The development of metallic coatings, such as Ni-Co alloys, with a particular emphasis on their homogeneity, processability, and sustainability, is of utmost significance. To address these challenges, the utilization of phenylsalicylimines (PSI) as additives within deep eutectic solvents (DES) was investigated, assessing their influence on the electrodeposition process of these metals at an intermediate temperature of 60°C, while circumventing aqueous reaction conditions. The findings demonstrated that the incorporation of PSI markedly enhances coating uniformity, resulting in an optimal cobalt content of 37% and an average thickness of 24 µm. Electrochemical evaluations revealed improvements in charge and mass transfer, thereby optimizing process efficiency. Moreover, computational studies confirmed that PSI forms stable complexes with Co(II), modulating the electrochemical characteristics of the system through the introduction of the diethylamino electron-donating group, which significantly stabilizes the coordinated forms with both components of the DES. Additionally, the coatings displayed exceptional corrosion resistance, with a rate of 0.781 µm per year, and achieved an optimal hardness of 38 N HRC, conforming to ASTM B994 standards. This research contributes to the development of electroplating bath designs for metallic coating deposition and lays the groundwork for the advancement of sophisticated technologies in functional coatings that augment corrosion resistance and mechanical properties.

Electrochemical assessment (Page 7 line: 250-258)

Changes in the functional groups of phenylsalicylimines alter the charge and mass transfer mechanisms, modifying the reduction rate of nickel and cobalt [52]. It is presumed that the amino group increases the hydrogen bond network and improves the selective complexation capacity of cobalt in ethaline base DES, which is reflected in the solvation and reduction of metal ion transport to the interfacial layer [53-55]. In contrast, the ethylamine functional group leads to the formation of a secondary amine that can coordinate with cobalt and nickel ions, while also influencing the overall charge distribution. As a result, this affects charge migration, transport, and transfer processes, but exhibits a greater response in cobalt oxidation as the current density decreases compared to nickel [56-58]. This preliminary analysis of the interaction between Phenylsalicylimines and ions was further examined through computational calculations, as elucidated below.

Conclusions (Page 14 line 476 to 495)

Molecular calculations reveal that the phenylsalicymines ligands with a diethylamino functional group (PSI-2) perform two primary roles. It forms stable species with a geometry increasing cobalt's electrostatic attraction to the cathode and improves its mobility by altering solvation and dipole moment. By coordinating with Co(II), it transfers charge, creating positively charged sites that facilitate electrode reactions. The increased dipole moment of Co(II) in square-planar complexes enhances its diffusion to the cathode, using positively charged areas as anchors, and facilitates alloy formation at the Ni-Co.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have made improvements to the manuscript based on the previous comments, which is appreciated. However, the electrochemistry section is not clearly presented and contains several inaccuracies. This part of the manuscript requires significant revision and correction to meet the standards for publication in Coatings. My detailed comments regarding the electrochemical analysis are provided below.

1. The CV discussion says the electrochemical processes are irreversible. But in the CV, both the reduction and oxidation peaks are observed, and the corresponding reactions are marked. What do you mean by irreversible here?
2. Also, the authors wrote, less energy is required for the deposition in ethaline. Need to compare these results with other electrochemical system and what is the usual overpotential required for reduction of Co/Ni?
3. The electrochemical reactions shown in figure 3 are correct? Is it reduction happening at negative potential and oxidation at anodic potentials? (eg., Co(II) +2 e gives Co?
4. Authors wrote, when PSI is added, “the current density decreases, leading to a reduction phenomenon that requires less energy”. How can the decrease in current density be related to less energy pathway for reduction? The decrease in current density indicates less charge passed, meaning less materials deposited, or a reduction in the total area available. Decrease in energy barrier is assessed by looking at the overpotential. Is there any decrease in overpotential (shift to positive potential) observed here. Authors need to make it clear.
5. “In oxidation, the effects of the functional group change in the PSIs’ can be observed, since for PSI-1, there is a potential shift from -0.2 V to 0.7 V and an increase in current output, which effectively reduces cobalt oxidation”.

The oxidation peak shift, as I can see in the CV, is from ~-0.1V to +0.4V. How the increase in current explains reduced cobalt oxidation? Increase in current should indicates increased cobalt oxidation? Did you compare the total charge during reduction and oxidation to see if there’s a significant mismatch?

6. “This is the first evidence suggesting that the -Et2N functional group acts as an additive in inhibiting the oxidation of metallic cobalt at the interface, creating sites that enhance interactions with delocalized electron species.”

The inhibition of oxidation should be reflected in the anodic peak currents, which displays clear peaks. Quantitative information should be gained by comparing the charge during reduction and oxidation. You observed reduced cathodic current after adding PSI-2 and similar reduction in the oxidation current, which might be matching.

7. The PSIs' action of Ni(II) increases in the diffusion-controlled phenomenon, widening the window and accompanied by an increase in current, which favors the nucleation and growth processes at the electrode-solution interface.”

Widening the potential window is not observed here, need clarification. The increase in current is not true in all cases. A visible increase is observed only when 0.5mM PSI is added. This need to be clarified.

8. “At the oxidation peak, there is an increase in current, which could be assumed to mean that the compound favors the passivation of the deposited nickel.” Again this is not true, based on figure 3 c.
9. “As the scan progressed with a more positive potential, a significant reduction in current was observed, predictably related to the decrease in solvent.” What is meant by decrease in solvent? What happens to solvent at more positive potential?
10. “Dissolutions were used with 0.5, 1, and 1.5 equivalents of ligand relative to cobalt, and were agitated at room temperature for 1 hour to assess the coordination formation of the compound.” Dissolutions?

 

 

 

Author Response

Thank you for the prompt second review, particularly for your excellent comments and patience in highlighting errors in fundamental concepts and language. The response to the comments is outlined below.

Comment 1: The CV discussion says the electrochemical processes are irreversible. But in the CV, both the reduction and oxidation peaks are observed, and the corresponding reactions are marked. What do you mean by irreversible here?

Response 1: In cyclic voltammetry, irreversible reactions are characterized by the electrochemical reaction not being in equilibrium and the kinetics of the electron transfer process being slow compared to the experiment's scan rate. But it should be written as quasi-reversible. The wording was changed as shown below. (Page 5., line 214-216)

Cobalt and nickel solutions exhibit reduction and oxidation peaks that vary due to the quasi-reversibility of the reactions, indicating that less energy is required to reduce the ions than to oxidize them. (Figure S3, Supplementary Information)

Comment 2: Also, the authors wrote, less energy is required for the deposition in ethaline. Need to compare these results with other electrochemical system and what is the usual overpotential required for reduction of Co/Ni?

Response 2: We agree that, based on the wording, it is understood to be a comparison; therefore, we removed it to avoid confusion for the reader.

Comment 3: The electrochemical reactions shown in figure 3 are correct? Is it reduction happening at negative potential and oxidation at anodic potentials? (eg., Co(II) +2 e gives Co?

Response 3: These are half-cell electrochemical reactions, and the nomenclature Co(II) is used because the cobalt ion is coordinated with the solvent, in comparison with water, which would be present as ions such as Co²⁺. However, if you prefer, you can change the nomenclature. Regarding the negative anodic potential, since the reference electrode is a non-aqueous silver nitrate solution, which has a potential of 0.36 V versus the ENH, a graph has been constructed with the conversion to ENH. This allows visualization that the oxidation peaks occur at positive potentials, while the reduction peaks are observed at negative potentials. (image link https://drive.google.com/open?id=1QXSD-J5zI8xhzzKrHCJKX3c6QEZTxyQS&usp=drive_fs)

Comment 4: Authors wrote, when PSI is added, “the current density decreases, leading to a reduction phenomenon that requires less energy”. How can the decrease in current density be related to less energy pathway for reduction? The decrease in current density indicates less charge passed, meaning less materials deposited, or a reduction in the total area available. Decrease in energy barrier is assessed by looking at the overpotential. Is there any decrease in overpotential (shift to positive potential) observed here. Authors need to make it clear.

Response 4: The descriptive interpretation of these phenomena can be related to phenomena such as charge transfer, mass adsorption, etc. It is a wording issue and has already been changed as shown below. (Page 6, line 228-237)

In the direct CV scan direction, the reduction peak located in the potential range of −0.4 V to −0.8 V is related to the process associated with the reaction Co (II)_DES + 2e- → Co(s). Meanwhile, during the anodic sweep, the oxidation peak, located in the potential range of −0.3 V to 0.1 V, is related to the dissolution of the deposited metal (from Co⁰ to Co²⁺). [46,47].

When PSIs’ are added, there are no changes in the potential window; however, the current density decreases on the reduction peaks, indicating that it requires less energy[48]. The change in the shape of the anodic sweep of the CVs is associated with the effects of the functional group change in the PSI’s. PSI-1 exhibits a potential shift from -0.2 V to 0.5 V, indicating an effective decrease in cobalt oxidation and, consequently, an increase in the process output current. For PSI-2, there is no apparent change in potential, but the oxidation current decreases. The cathodic current is suppressed, indicating slower nucleation and growth of cobalt due to interactions between cobalt and the ligand. PSI-1 and PSI-2 can be adsorbed onto the electrode surface, inhibiting Co deposition and thereby reducing the growth and nucleation of Co, which increases the nucleation rate.

Comment 5:“In oxidation, the effects of the functional group change in the PSIs’ can be observed, since for PSI-1, there is a potential shift from -0.2 V to 0.7 V and an increase in current output, which effectively reduces cobalt oxidation”. The oxidation peak shift, as I can see in the CV, is from ~-0.1V to +0.4V. How the increase in current explains reduced cobalt oxidation? Increase in current should indicates increased cobalt oxidation? Did you compare the total charge during reduction and oxidation to see if there’s a significant mismatch?

Response 5: We agree with the comment. I am very embarrassed that a wording issue led to the analysis being misinterpreted. It has now changed. I apologize. The changes are shown below. (Page 6 line 228-237)

In the direct CV scan direction, the reduction peak located in the potential range of −0.4 V to −0.8 V is related to the process associated with the reaction Co (II)_DES + 2e- → Co(s). Meanwhile, during the anodic sweep, the oxidation peak, located in the potential range of −0.3 V to 0.1 V, is related to the dissolution of the deposited metal (from Co⁰ to Co²⁺). [46,47].

When PSIs’ are added, there are no changes in the potential window; however, the current density decreases on the reduction peaks, indicating that it requires less energy[48]. The change in the shape of the anodic sweep of the CVs is associated with the effects of the functional group change in the PSI’s. PSI-1 exhibits a potential shift from -0.2 V to 0.5 V, indicating an effective decrease in cobalt oxidation and, consequently, an increase in the process output current. For PSI-2, there is no apparent change in potential, but the oxidation current decreases. The cathodic current is suppressed, indicating slower nucleation and growth of cobalt due to interactions between cobalt and the ligand. PSI-1 and PSI-2 can be adsorbed onto the electrode surface, inhibiting Co deposition and thereby reducing the growth and nucleation of Co, which increases the nucleation rate.

 

Comment 6:“This is the first evidence suggesting that the -Et2N functional group acts as an additive in inhibiting the oxidation of metallic cobalt at the interface, creating sites that enhance interactions with delocalized electron species.”

Response 6: We agree that this paragraph does not contribute to a scientific analysis and is more speculative. It has been removed because there is no scientific evidence to support it.

Comment 7:The inhibition of oxidation should be reflected in the anodic peak currents, which displays clear peaks. Quantitative information should be gained by comparing the charge during reduction and oxidation. You observed reduced cathodic current after adding PSI-2 and similar reduction in the oxidation current, which might be matching.

Response 7: We agree with the comment and once again apologize for the poor wording, which has been modified to avoid confusion, as shown below. (Page 6, line 228-239 and Page 7, line 240-246)

When PSIs’ are added, there are no changes in the potential window; however, the current density decreases on the reduction peaks, indicating that it requires less energy[48]. The change in the shape of the anodic sweep of the CVs is associated with the effects of the functional group change in the PSI’s. PSI-1 exhibits a potential shift from -0.2 V to 0.5 V, indicating an effective decrease in cobalt oxidation and, consequently, an increase in the process output current. For PSI-2, there is no apparent change in potential, but the oxidation current decreases. The cathodic current is suppressed, indicating slower nucleation and growth of cobalt due to interactions between cobalt and the ligand. PSI-1 and PSI-2 can be adsorbed onto the electrode surface, inhibiting Co deposition and thereby reducing the growth and nucleation of Co, which increases the nucleation rate.

Yes, the calculations were made,  for PSI-1   width (1/2) :  = 0.121 5 V, Ep-Ep/2 = 0.08018 V, positive charge = 0 mC, negative charge = -0.4566 mC , position = 0.601 5 V   height = 0.154 8 mA   width (1/2) :  = 7.452e-3 V Ep-Ep/2 = 5.883e-3 V charge pos. = 0.046 13 mC charge neg. = -68. 17 mC, only excluded because the article would be too long.

Comment 8:The PSIs' action of Ni(II) increases in the diffusion-controlled phenomenon, widening the window and accompanied by an increase in current, which favors the nucleation and growth processes at the electrode-solution interface.”

Widening the potential window is not observed here, need clarification. The increase in current is not true in all cases. A visible increase is observed only when 0.5mM PSI is added. This need to be clarified.

Response 8: Thank you for your valuable comment. It has been included in the text to make the analysis much more straightforward and avoid any ambiguity. A zoom of the nickel reduction peak was included in Figure 3C to illustrate the description in the text. (Page 6, line 219)

Comment 9:“At the oxidation peak, there is an increase in current, which could be assumed to mean that the compound favors the passivation of the deposited nickel.” Again this is not true, based on figure 3 c.

Response 9: You are right, the wording has been changed as shown below. (Page 7, line 242-245)

At the oxidation peak, a current increase occurs only at a concentration of 0.5 mM, which suggests that the compound promotes the passivation of nickel deposited at the highest concentration evaluated.

Comment 10:“As the scan progressed with a more positive potential, a significant reduction in current was observed, predictably related to the decrease in solvent.” What is meant by decrease in solvent? What happens to solvent at more positive potential?

Response 10: I apologize; this translation is inaccurate. It pertains to electrolyte decomposition. The wording has been adjusted as demonstrated below. (Page 7, line 245-247)

As the reverse scan advanced, a noteworthy decrease in current was observed, which is predictably associated with the electrolysis decomposition of the solvent.

Comment 11:“Dissolutions were used with 0.5, 1, and 1.5 equivalents of ligand relative to cobalt, and were agitated at room temperature for 1 hour to assess the coordination formation of the compound.” Dissolutions?

Response 11: We agree with the comment. The wording has been changed to improve the explanation of UV Vis measurements and the interpretation of results, as shown below. (Page 7, line 261-265)

The SPIs´ ligand concentration was tested at 0.5, 1, and 1.5 equivalents in the cobalt solution with Ethaline to clarify the interaction between the electrolyte components in the baths more effectively. The measurements were taken after the sample had remained under agitation at room temperature for one hour.

 

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript has been improved and addressed the majority of my previous comments. Below are two points which needs attention befoere publishing in 'coatings'.

1. The electrochemical reactions written under the cathodic and anodic peaks are correct? (Figure 3). Cathodic process is Co(II) + 2e giving Co and anodic is the opposite. Its written Co(II) giving Co+2e.

2. “the current density decreases on the reduction peaks, indicating that it requires less energy”. I am still not convinced that the decrease in current density is related to less energy deposition. I couldn't find clear evidence for it in the reference given (48?).

Author Response

The authors would like to express their gratitude to the reviewer for their valuable comments, which have significantly enriched the manuscript. Below, we include the response to the reviewer's comments, along with the revised and modified sections of the manuscript. The changes made are highlighted in the updated manuscript.

Comment 1. The electrochemical reactions written under the cathodic and anodic peaks are correct? (Figure 3). Cathodic process is Co(II)_(DES) + 2e^-  Co and anodic is the opposite. Its written Co(II) giving Co+2e.

Response 1: I apologize for the oversight. I did not realize what a terrible mistake it was. Figure 3 has now been corrected. I appreciate your patience in pointing it out to me without taking offense. Thank you very much.

Comment 2. “the current density decreases on the reduction peaks, indicating that it requires less energy”. I am still not convinced that the decrease in current density is related to less energy deposition. I couldn't find clear evidence for it in the reference given (48?).

Response 2: We agree the term is imprecise; a decrease in CV reduction peak current suggests slower deposition, not lower energy for reduction. It relates to changes in the reduction potential needed to overcome activation barriers. The wording was revised to prevent technical inaccuracies, and new references were added. As shown below (Page 6, lines 228 to 232)

Adding PSIs to the cobalt DES solution does not show potential window changes; however, a decrease in current density at reduction peaks was observed, indicating that the rate of the electrochemical reaction, specifically the reduction of cobalt ions in this case, is slower. This suggests a change in the kinetics of the process, likely due to factors like increased resistance or slower electron transfer at the electrode surface [48-52]

References

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