Preparation of Vanadium (3.5⁺) Electrolyte by Hydrothermal Reduction Process Using Citric Acid for Vanadium Redox Flow Battery

Comments 1: Redraw Figure 1 to ensure its content does not duplicate other papers. Clearly cite the source or provide original data charts.

DOI: 10.1002/aenm.201100008

Response 1: ①I have revised the references in my manuscript where is 45 lines in 2page.

            ② References were inserted on page 12, lines 382-384

Comments 2: Elaborate on the advantages of vanadium (3.5+) electrolyte compared to traditional electrolytes (VO₂⁺/VO₂⁺ and V³⁺/ V²⁺), such as its benefits in electrochemical performance, stability, or cost.

Response 2:

1) Seeing the electrochemical performance, the electrolyte produced using the hydrothermal reduction process demonstrated higher efficiency compared to the electrolyte manufactured by conventional methods. This is because mixing vanadium (3+) and vanadium (4+) electrolytes using an electrochemical cell can lead to difficulties in achieving consistent battery efficiency and raises concerns about the aging of internal cell components.

 2) When examining stability, the hydrothermal reduction process demonstrates higher stability because it is simpler and allows for the production of a consistent electrolyte on a large scale. In contrast, the unit cell method involves a more complex process, making it challenging to maintain the stability of the electrolyte.

 3) In the hydrothermal reduction process, citric acid is generally inexpensive, and the cost of raw materials can be adjusted according to the concentration of citric acid used in the process. However, the unit cell method for producing electrolytes is less cost-effective due to the complexity of the process.

Comments 3:  In sections 2.2 and 2.3, clearly explain the purpose and significance of each part of the experimental design, and indicate the corresponding results and discussions. Explain why Vanadium (IV) is not directly mixed with Vanadium (Ⅲ), but rather oxidized to Vanadium (V) and then reduced to Vanadium (IV).

Response 3:

①    The purpose of the experimental design in Sections 2.2 and 2.3 was to compare the electrochemical characteristics and performance of electrolytes produced by adding reducing agents like oxalic acid to VOSO₄ and V₂O₅ precursors with those produced through the hydrothermal reduction process.

②    The importance of the experimental design lies in the use of VOSO₄ as a precursor to produce vanadium electrolyte due to its high solubility and excellent stability, making it a reference material for vanadium electrolytes. Additionally, V₂O₅ is used despite its lower solubility because it is approximately four times cheaper than VOSO₄ and is commercially utilized as an electrolyte. In conclusion, the comparison of VOSO₄ and V₂O₅ aims to understand the advantages and disadvantages of each, thereby providing foundational data for the development of improved vanadium electrolytes.

③     When vanadium (4+) is charged, vanadium (5+) is produced at the positive and vanadium (3+) is generated at the negative. Since the electrolyte for charging and discharging needs to be vanadium (3.5+), vanadium (5+) is reduced to vanadium (4+), and then mixed in equal amounts with the vanadium (3+) electrolyte. This process, however, takes a significant amount of time when preparing the electrolyte using a single cell.

Comments 4: Provide detailed reasons for studying the synthesis of vanadium (3.5+) electrolyte using materials VOSO₄ and V₂O₅, and explain the differences between electrolytes prepared with different reducing agents. 

Response 4:

1) To store and release energy through efficient and stable electrochemical reactions in energy storage systems such as Vanadium Redox Flow Batteries (VRFBs).

2)The electrolyte produced using citric acid has higher reducing power than that made with oxalic acid, resulting in improved electrolyte stability.

3) Oxalic acid is a strong reducing agent, so it oxidizes quickly and affects the stability of the reduced electrolyte.

Comments 5: Clearly explain the content of Figure 5. What raw materials were used to prepare the vanadium (3.5+) electrolyte?

Response 5:

1) In Figure 5, (a) shows the electrolyte produced by reducing the precursor V₂O₅ to vanadium (4⁺) using citric acid, while (b) depicts the production of vanadium (3.5⁺) electrolyte HRR of the electrolyte from (a).

2) The raw material is V₂O₅

Comments 6: In Figure 6, Vanadium (Ⅲ) shows two peaks in the UV spectra. Describe in detail how to calculate and fit the relationship between Vanadium (Ⅲ) concentration and peak intensity based on these two peaks.

Response 6:

Comments 7: The results of Figures 7a and 7b seem to indicate the successful preparation of vanadium (3.5+) electrolyte, especially at an OA concentration of 6.4 M, where vanadium (IV) and vanadium (III) are clearly present. Explain the reason for choosing citric acid (for preparing vanadium (3.5+) electrolyte.:

Response 7:

Citric acid was chosen because it has a higher reducing power than oxalic acid and its molecular structure allows it to transfer four electrons to external substances, enabling direct conversion to vanadium (3.5+).

Comments 8: The author proves that the detection method for the successful preparation of vanadium (3.5+) electrolyte is too simple, and suggests using quantitative analysis methods to accurately determine the concentrations of Vanadium (IV) and Vanadium (Ⅲ) in the electrolyte, such as spectrophotometry, atomic absorption spectroscopy (AAS), inductively coupled plasma mass spectrometry (ICP-MS).The instruments and analytical methods used can refer to the following paper, which is of reference value to you and worth citing.

https://doi.org/10.1016/j.jwpe.2024.105415

https://doi.org/10.1016/j.jhazmat.2023.132219

Response 8:

I agree with this comment. And thank you for providing the reference papers.

But The concentration of an electrolyte can be precisely using UV absorbance values.

 Later, I will definitely look into it  

Comments 9: Clearly state the advantages of the hydrothermal method for preparing vanadium (3.5+) electrolyte, and provide experimental data to prove its superiority over traditional processes, such as cost-effectiveness.

Response 9:

1)The hydrothermal reduction process is relatively simple, suitable for large-scale production, and the resulting electrolyte has more stable redox reactions. Additionally, it allows for easier adjustment of the electrolyte composition.

2) Looking at the efficiency values of the unit cell in Table 2 on page 11 (lines 341–342), it can be determined that the performance is superior.

Comments 10: Supplement the description of the specific electrolyte used in the VRFB cell test.

Response 10:

 I have revised and supplement section 3.3 of the manuscript (page 10-11) and provided explanations for the cell test figures of the three electrolytes.

 Firrst ; 1.6M VOSO₄ + 3M H₂SO₄ + OA (by conventional method)

Second : 1.6M VOSO₄ + 3M H₂SO₄ + OA (by HRR)

Third: 0.8M V₂O₅ + 3M H₂SO₄ + CA (by HRR)

Comments 11: Supplement the experiments on the effect of environmental temperature changes on the stability of vanadium (3.5+) electrolyte.

Response 11:

Figure 4(b) illustrates the process of producing vanadium (3.5+) using the hydrothermal reduction method. In this process, the reactor is placed in the furnace, and the temperature is increased at a rate of 5°C/min until it reaches 150°C, at which point the electrolyte is formed.

4. Response to Comments on the Quality of English Language

Point 1:

Response 1:    (in red)

5. Additional clarifications

[Here, mention any other clarifications you would like to provide to the journal editor/reviewer.]

Comments 1: The author should review the manuscript and correct some typographical errors. For example, in line 95, the author writes "V3.5+" whereas in the rest of the text it appears as "3.5⁺". In line 154, "reductio" should be corrected to "reduction". In other sections, "X ºC" appears instead of "XºC". It is recommended to carefully read through and address these types of errors.

Response 1: Thank you for pointing out the typo

         I have corrected "3.5+" to "V (3.5⁺)" on line 95 and "reductio" to "reduction" on line 154.  

Comments 2: Regarding the experimental section, why does the author not employ an experimental design to improve the electrochemical response of the electrode? On what basis does the author decide the concentrations of citric acid used in the reaction? Similarly, the temperatures? It would be interesting to know how these variables interact with each other and optimize the electrode's electrochemical response.

Response 2:

①In the experimental sections 2.2 and 2.3, the V(III) electrolyte is prepared at the negative and the V(V) electrolyte at the positive through charging. The produced V (3+) is then mixed with the reduced V (4+) to create a V (3.5+) solution, which is used to conduct charge-discharge tests to evaluate performance. These efforts were aimed at improving the electrochemical reactions at the electrodes.

② The concentration of citric acid was determined by analyzing with the CV equipment. As it showed a decrease in the peak of vanadium (4+) around 750 nm and an increase in the peak of vanadium (3+) around 400 nm, The 1.6M citric acid graph was found to be the most suitable.

③ The reaction temperature in the reactor was raised to 150°C because the melting point of citric acid is around 153°C.

Comments 3: The graphs in Figure 6 need improvement. It is suggested to use a screenshot (Figure 6a) instead of a cellphone capture. In Figure 6b, the axes of the graphs are not legible. Why does the author use a graphics editor for the subsequent UV-vis figures but a cellphone capture for Figure 6a

Response 3:

The attached document contains the concentration measurements taken from the UV equipment. Unfortunately, I was unable to locate the raw data, so I captured and uploaded the printed output. I will continue searching for the raw data and make the necessary revisions and updates.

Comments 4: In Figure 8, why is the current referenced to the area in some cases (Figure 8a) but not in others (Figure 8b)?

Response 4: The current density was missing, so I have revised it

Comments 5: Review the manuscript for grammatical accuracy and consistency in formatting, particularly in the introduction, and, the Result and Discussion sections.

Response 5: I have revised the manuscript to correct grammatical accuracy and improve consistency in formatting.

Comments 1: Novelty of this work should be highlighted in the abstract.

Response 1:

 There are no existing research papers on the production of vanadium electrolytes using citric acid in the sequential reduction process with V2O5, so I believe this paper has originality, as expressed in the abstract.

Comments 2: Figure 4, Figure 6 should be revised. It is very difficult to read the text in the figures.

Response 2: I have revised the text in Figures 4 and 6 to improve readability.

Comments 3: Figure 8. needs to be changed. It is very difficult to understand the information from author and why the current units are different in two figures.

Response 3: The current density was missing, so I have revised it

Comments 4: Why is the expression of VE (%) written in twice in lines 205 and 206? What is average chatge voltage (V)?

Response 4:

I have corrected the typo. Since the voltage (potential) is not constant, the average value is used.

Comments 5: Electrochemical impedance spectroscopy (EIS) studies with proper explanation need to be provided.

Response 5:

EIS analysis was conducted on the three manufactured electrolytes, and the explanation is provided on page 9, line 291.

Comments 6: References are not written correctly in the manuscript. Most of the references are very old. The author can include some recent references in the manuscript.

Response 6:

I will find and revise the references with recent ones.

Comments 7: A comparison table of electrochemical performance of RFBs should be included in the manuscript

Response 7:

On pages 11, lines 341-342, the voltage efficiency, current efficiency, and energy efficiency are detailed. I will add any missing information and submit the revised content accordingly.

Comments 1: A minor revision is still required before final acceptance. The EIS curves in Figure 8d are wrongly plotted. It should be checked and modified.

Response 1: I have checked and revised figure 8d.  It was shown in line 300-301

Additionally, I will continue making improvements to the English throughout the manuscript.