Synthesis and Evaluation of Layered Ni–Co and Ni–Co–Ni Electrodes Modified by Molten–Salt Al Deposition/Dissolution Technique for Electrochemical Applications

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Major Comments

Reproducibility and statistics
The manuscript does not state how many samples were tested per condition. Electrochemical measurements (especially Tafel slopes and chronopotentiometry) should include error bars or standard deviations. Please add this information.

Tafel slope interpretation
The Ni-Co electrode shows the highest current at −0.6 V but a Tafel slope (166 mV dec⁻¹) higher than porous Ni (128 mV dec⁻¹). This is unusual and deserves more discussion. Could this be due to mass transport limitations or different rate-determining steps? The current explanation (“mainly related to strongly developed porous interface”) is somewhat vague.

Missing control experiment
A porous Co-only electrode (or Co-Al derived) would help clarify whether the Co-rich surface alone is responsible for the high current density of Ni-Co, or whether the synergy with Ni is essential.

Phase identification
In Fig. 5A (Ni-Co after Al deposition), the main peak is assigned to Al₃Co (111). However, this peak overlaps closely with Ni (111) at ~44.5°. Given the EDS shows Ni still present, a more careful peak deconvolution or additional XRD with longer scans is recommended.

Figure references
Several figures are referred to as “Fig. xx” (e.g., page 11, line 399). Please correct these.

Minor Comments

Typographical/grammatical issues

Page 7, line 267: “It also need to be underlined” → “It should also be underlined”

Page 8, line 282: “Interesting point is that, for post-deposited Al sample” → “An interesting point is that, for the post-deposited Al sample”

Page 10, line 378: “corall or sponge-like” → “coral-like or sponge-like”

Terminology
“Ni-Co-Ni” is clearly defined, but in the abstract and conclusions, “Ni-Co-Ni” appears next to “Ni-Co”. Consider using “Ni-Co (bilayer)” and “Ni-Co-Ni (sandwich)” consistently for clarity.

Molten salt composition
The melt is NaCl-KCl-AlF₃ (3.5 mol%). Why 3.5 mol%? Is this optimal for Al deposition? A brief justification would help.

Reference electrode
The Ag/AgCl reference electrode used at 750°C is unusual. How was it constructed and calibrated? Provide more details or a citation.

Chronopotentiometry stability
The stability tests are only 2 hours. For practical applications, 24–100 h tests are typical. Please comment on this limitation.

Author Response

Dear Reviewer,

 

our answers for your revision are below and in attachment.  

 

Best regards,

 

Revision letter for Reviewer 1.         

 

• Reproducibility and statistics

The manuscript does not state how many samples were tested per condition. Electrochemical measurements (especially Tafel slopes and chronopotentiometry) should include error bars or standard deviations. Please add this information.

Answer:

Thank you for mentioning this issue. In term of the sample number – each sample have been prepared several times, due to the fact in our investigations we followed the XRD and EDS/Mapping evolution of cross sections for each stage of molten salts modification technique.

For electrochemical investigation we selected one sample for Ni-Co and Ni-Co-Ni systems and they were tested in LSV tests and CP in 1 M NaOH solutions. Taking into consideration our expertise with other systems like Ni, stainless steel, Ni-Pt, Ni-Pt-Ni, Ni-Ir – the reproductibility is preserved.

Due to our expertise, we do not seen any Tafel slope graphs with error bars or any standard deviation estimation. In our case, the Tafel slope were determined by the linnear regression between linnear range determined in a current density range from 1 to 10 mA cm². This practice is quite common in scientific literature.  In term of chronopotentiometric measurements, the estimated values were taken from the stable range of registered potential, which is a good practice in catalytic activity estimation.

• Tafel slope interpretation

The Ni-Co electrode shows the highest current at −0.6 V but a Tafel slope (166 mV dec⁻¹) higher than porous Ni (128 mV dec⁻¹). This is unusual and deserves more discussion. Could this be due to mass transport limitations or different rate-determining steps? The current explanation (“mainly related to strongly developed porous interface”) is somewhat vague.

Answer:

This observation is very important and we want to thank the reviewer for pointing out this issue. Generally, the scientific literature suggests that the metallic Co exhibit lower exchange current denstity than Ni in HER. Moreover, the reported catalytic activity for pure Co is lower than Ni, what we also observed in our work.

For Ni-Co system the composition of interface area is mostly Co – which exhibit lower catalytic activity than Ni (in terms of general comparison with Ni). When we performed the EDX/mapping  for Ni-Co-Ni sandwich system, the most of interface area is a porous Ni structure – and additionally, taking into account the morphological issues from SEM observations and cross section, the surface is significantly more developed – what is already stated in manuscript. So that is the general explanation of such a pronounced differences in term of catalytic activity.

The corrected text is as follows:

Interestingly, the porous Ni-Co electrode, despite showing the highest cathodic current density at large negative polarisation, exhibited a Tafel slope of 166 mV dec⁻¹, which is higher than that of porous Ni. From literature point of view, the Tafel slope for Ni should be lower than for Co, what is related to better catalytic activity of Ni electrodes. Simmilar trend can be observed for porous electrodes, where kinetics for Ni is better than for NiCo bilayer. This can be attributed to the fact the interface area is mostly a pure porous Co. But overall catalytic activity, measured as a current density for fixed potential is higher for bilayer and indicates that the superior activity of the Ni-Co electrode. In contrast, the sandwich-type Ni-Co-Ni architecture showed the lowest Tafel slope among the porous materials, equal to 111 mV dec⁻¹. These phenomenon can be explained by superposition of the kinetical issue related to Ni-rich interface. These findings suggests that the introduction of an additional Ni layer before molten-salt treatment modifies the final porous architecture and local chemical environment in a way that is a tool for tailoring the catalytic activity of electrodes.

• Missing control experiment

A porous Co-only electrode (or Co-Al derived) would help clarify whether the Co-rich surface alone is responsible for the high current density of Ni-Co, or whether the synergy with Ni is essential.

Answer:

We do not have any bulk Co electrode which can be modified by molten salts treatment. But taking into account the results for Ni and stainless steels (SUS304 and SUS316L) the formation of porous body is an main factor which increase the catalytic activity of electrodes.

• Phase identification

In Fig. 5A (Ni-Co after Al deposition), the main peak is assigned to Al₃Co (111). However, this peak overlaps closely with Ni (111) at ~44.5°. Given the EDS shows Ni still present, a more careful peak deconvolution or additional XRD with longer scans is recommended.

Answer:

Thank you for pointing out this issue. We detected a significant missing, on the Ni-Co binary electrode after molten salts treatment. Careful investigation of registered peaks were done and the position of peaks (yellow XRD – top one) reveal the other positions which we mentioned in submitted manuscript: 31.34, 44.71, 65.27 and 82.79. Careful screening of Ni-Co, Ni-Al, Co and Co-Al phases in JCPDS database reveal a perfect match of all these signals for β-AlCo phase ( JCPDS 44-1115) for orientations respectively (100), (110), (200) and (220). Moreover, some residual Al signals were observed by EDX/mapping of cross section, which can explain the presence of signals. It should be noted that the penetration depth of X-ray under our condition is approximately 5-7 micrometres (depend from the examined coatings). So the top layer of surface could be covered by non-dissolved Co-Al phase.

Corrected XRD diffraction patterns are included into revised version of the manuscript with a corrected text as follows:

We also performed an XRD of the same sample after HER/OER and the diffraction is free of the Co-Al small reflexes – only fcc-Co signals, what can be seen on additional figure, inserted only into this revision letter. We prefer to leave the previous XRD spectra as it was, with only corrected Co and Co-Al residual phases.

It should be also pointed out that before the electrochemical investigations (ECSA, LSV, CP) the electrode were immersed into 1 M NaOH for stabilization of OCP. These conditions also led to dissolution of remaining traces of Al (due to the amphoteric character of Al).

• Figure references

Several figures are referred to as “Fig. xx” (e.g., page 11, line 399). Please correct these.

Answer:

Thank you for mention this issue. We mislead it during a manuscript screening.

• Typographical/grammatical issues

Page 7, line 267: “It also need to be underlined” → “It should also be underlined”

Page 8, line 282: “Interesting point is that, for post-deposited Al sample” → “An interesting point is that, for the post-deposited Al sample”

Page 10, line 378: “corall or sponge-like” → “coral-like or sponge-like”

Answer:

We are really thankful for identifying these language issues. We really appreciate your efforts to make our work more professional in term of wording.

 

• Terminology

“Ni-Co-Ni” is clearly defined, but in the abstract and conclusions, “Ni-Co-Ni” appears next to “Ni-Co”. Consider using “Ni-Co (bilayer)” and “Ni-Co-Ni (sandwich)” consistently for clarity.

          Answer:

This differentiation of the structure organization is now more underlined in manuscript. Thank you for this suggestion.       

• Molten salt composition

The melt is NaCl-KCl-AlF₃ (3.5 mol%). Why 3.5 mol%? Is this optimal for Al deposition? A brief justification would help.

Answer:

Yes, this type of molten salts electrolyte is quite common choice in term of Al electrochemistry. Selected concentration of Al ensure the stable and uniform deposition of Al on the electrode surface.  We and team of our Japanese collaborators did many works related to deposition of Al onto different substrates, which simulated this highly- corrosive molten Al contact with metallic layers. These formulation and experimental conditions were selected based on our experience and previously published works in Ni, Ni-Pt, Ni-Ir and steel-based systems.

• Reference electrode

The Ag/AgCl reference electrode used at 750°C is unusual. How was it constructed and calibrated? Provide more details or a citation.

Answer:

This is a tested and widely used reference system for molten salts electrochemistry. We prepare this type of reference as a mullite rod with Ag wire and AgCl powder which is inserted into the molten salts. Our system is stable and tested in many different scientific approaches.

• Chronopotentiometry stability

The stability tests are only 2 hours. For practical applications, 24–100 h tests are typical. Please comment on this limitation.

Answer:

We do not performed a long-time experiments. From our point of view the 2 hour testing is enough to underline a stability and general difference between selected architectures in term of catalytic activity. The word “long term” from manuscript has been removed. Thank you for pointing out this issue.

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Dear Reviewer, authors are very thankful for all suggested changes, modifications and detected flaws in our manuscript. We really appreciate your effort to improve the scientific quality of our work.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

It is suggested to revise the manuscript. 

1. Molten‑salt Al treatment should be discussed and its advantages over other methods.
2. What is mechanism for the formation of Ni–Co with open coral‑like network?
3. The 111 mV dec⁻¹ Tafel slope value should be compared with Ir or Ru based system.
4. What is the effect of precursor on the porosity?
5. Figure 1 should be improved.
6. Figure 6, SEM images should be improved with proper scale.
7. Can you provide HRTEM for porosity confirmation.
8. Please provide XPS and BET in details for further confirmation.
9. Discuss the mechanism for electrochemical reaction. 

Author Response

Dear Reviewer,

Our response letter can be found in attachment and below as a text. 

Best regards,

 

 

• Molten‑salt Al treatment should be discussed and its advantages over other methods.

Answer:

This work introduces a novel fabrication method for porous nickel–cobalt (Ni–Co) and layered nickel–cobalt–nickel (Ni–Co–Ni) electrodes using a molten-salt Al modification technique.

To our best knowledge, the application of molten salts technique, especially in term of layered structures like Ni-Co and Ni-Co-Ni were not investigated in scientific literature and can provide a valuable insights into development of highly-active materials for catalytic applications.

Generally speaking, the molten salts modification technique, which is a crucial point of our research collaboration, based on the high-temperature molten salts deposition of Al on metallic surface. Selected temperature (750oC) allow to keep the deposited Al in liquid state, which makes the intediffusion process significantly easier. Based on the thermodynamic data, the Gibbs free entalphy formation of Co-Al and Ni-Al phases at selected temperature is quite negative (around -143 kJ/mol) what means that this reaction is spontaneous. So application of cathodic potential led to deposition of Al which react with Co and Ni on the interface region. Furhermore, the modification of potential towards more positive is dissolving Al and Al-based phases, revealing the porous structure. This mechanism were tested in our works for Ni, Co, Ni-Pt, Ni-Pt-Ni layered structure, Ni-Ir system and stainless steels. All of them exhibit exactly the same mechanism of intemetallic phases formation.

The advantage of the molten salts modification treatment over other methods is related to its versatility in term of applied modified materials. We can obtain a porous structures of metals, layered structures and alloys. Adjusting the temperature and deposition potential is a tool for projecting the thickness and pore distribution over a porous interface. Other techniques are not able to create such a well-developed micro and nanoporous metallic network.

 

• What is mechanism for the formation of Ni–Co with open coral‑like network?

Answer:

The Ni-Co bilayer system is significantly different from the Ni-Co-Ni system. The formation of intermetallic phases of Co and Al have much higher spontaneous characteristics than for Ni, what is related to higher Gibbs formation entalphy for Co-based one. It means that the reaction during dissolution and the transport of the substrates can be significantly higher in this particular case.

 

• The 111 mV dec⁻¹ Tafel slope value should be compared with Ir or Ru based system.

Answer:

We do not have a raw data for Ir or Ru tested in this particular conditions (porous electrodes). But from the scientific literature we can get the information about that in 1 M NaOH these elements exhibit significantly lower Tafel slope values, close to 32 mV dec-1. Simmilar results we registered for Ni-Pt porous system obtained by molten salts modification technique, which were published in work as follows: 10.1016/j.ijhydene.2026.153450

 

• What is the effect of precursor on the porosity?

Answer:

In terms of precursor reviewer is referring to Al in molten salts? If yes, the Al is a crucial element in this technique. It need to be in a liquid state and chemically react with the surface of modified electrode by itself.

 

• Figure 1 should be improved.

Answer:

The figure 1 is currently modified. Thank you for your suggestions.

 

• Figure 6, SEM images should be improved with proper scale.

Answer:

Dear Reviewer, this scale were added only in term of visibility of bar scale. We strongly believe that current version is good because is bigger than an original picture.

 

• Can you provide HRTEM for porosity confirmation.

Answer:

Unfortunately we do not have in our department a possibility to do any TEM observation. But we do not know what would be an outcome from this observation. Formation of porous body is visible at such a low magnification with SEM images.

 

 

 

 

• Please provide XPS and BET in details for further confirmation.

Answer:

Dear Reviewer, the chemical composition of the interfacial area is well established by SEM/EDX/mapping. If we would modify the surface with noble metals like Pd, Pt, Ru,Rh by chemical route or decorate the surface with nanoparticles – yes, the XPS would be very interesting point – due to quite low penetration of this technique. For our quite thick layers, the XPS measurements seems to be a pointless.

In case of BET analysis – we tried to do it for our Ni porous electrodes, Ni-Pt and Ni-Pt-Ni, but this technique do not get us any logical and sensible results, that we do not perform it for this particular study also.

• Discuss the mechanism for electrochemical reaction.

Answer:

In our system we have only two electrochemical reactions:

 Al³⁺ + 3e à Al (liquid state)

Al à Al³⁺ + 3e  (or similar for Al-based phases).

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Dear Reviewer, authors are very thankful for all suggested changes, modifications and detected flaws in our manuscript. We really appreciate your effort to improve the scientific quality of our work.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The revised manuscript, in my opinion, is highly suitable for publication. 

Reviewer 2 Report

Comments and Suggestions for Authors

 -------- Accepted in present form