Enhanced Performance of WO₃/SnO₂ Nanocomposite Electrodes with Redox-Active Electrolytes for Supercapacitors

Round 1

Reviewer 1 Report

This manuscript reports the Enhanced Performance of WO3/SnO2 Nanocomposite Electrodes with Redox-Active Electrolytes for Supercapacitors. The electrochemical activities of WO3/SnO2 Nanocomposite Electrodes was investigated in K3Fe(CN)6. K3Fe(CN)6 electrolyte. WO3/SnO2 nanocomposite electrode demonstrated excellent specific capacitance. This manuscript needs following minor revision before publication:

1.      While comparing the electrochemical performance of any electrode, the electrochemical performance should be compared in similar operating conditions. The electrochemical performance of the WO3-based active electrode materials should be compared in redox active electrolytes.

2.      Authors should explain suitable reasons for enhancing the electrochemical performance of the WO3/SnO2 Nanocomposite Electrodes when K3Fe(CN)6 is added into a 1 M Na2SO4.

3.      Why the coulombic efficiencies of WO3/SnO2 symmetric device is very low?

4.      In WO3/SnO2 Nanocomposite electrode, which component has a major role in obtaining high-performance symmetric supercapacitors?

5.      The authors should incorporate some latest articles based on this work

https://doi.org/10.1016/j.cplett.2022.139884

 

https://doi.org/10.1021/acssuschemeng.1c08059

Author Response

We deeply thank to the Reviewer 1 for highly suggestive comments. We add our responses to the comments one-by-one.

 

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Comments and Suggestions for Authors

This manuscript reports the Enhanced Performance of WO₃/SnO₂ Nanocomposite Electrodes with Redox-Active Electrolytes for Supercapacitors. The electrochemical activities of WO₃/SnO₂ Nanocomposite Electrodes was investigated in K₃Fe(CN)₆. K₃Fe(CN)₆ electrolyte. WO₃/SnO₂ nanocomposite electrode demonstrated excellent specific capacitance. This manuscript needs following minor revision before publication:

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Comment 1

While comparing the electrochemical performance of any electrode, the electrochemical performance should be compared in similar operating conditions. The electrochemical performance of the WO3-based active electrode materials should be compared in redox active electrolytes.

 

Response 1

In Table 3, we compared our electrode to the other WO₃-based electrodes in different conditions the authors reported. To our knowledge, the effects of redox active electrolytes on these electrodes were not yet reported. For comparison with the WO₃-based electrodes, we used a WO₃ electrode prepared by ourselves as a “standard”. Then, the advantage of WO₃/SnO₂ nanocomposite electrode and the effects of redox additives were discussed. Yes, the other WO₃-based electrodes can work more in the presence of redox active electrolytes. Thus, we added short comments in the discussion and conclusion parts as below.

LINE 572: “The improved specific capacitance was also observed for single component electrodes (both of WO₃ and SnO₂). Therefore, this approach can be used for other electrodes using these metal oxides, although the reaction efficiency could be differed by the electrode species.”

LINE 602-: “Thus, the WO₃/SnO₂ nanocomposite electrode grown on carbon cloth is a promising candidates as binder free electrode for high performance of supercapacitor device, and the supercapacitor system using redox active K₃Fe(CN)₆ electrolyte was proposed. These strategies developed in this study can also be applied to other metal oxide nanocomposites to improve the performance of supercapacitors.”

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Comment 2

Authors should explain suitable reasons for enhancing the electrochemical performance of the WO₃/SnO₂ Nanocomposite Electrodes when K₃Fe(CN)₆ is added into a 1 M Na₂SO₄.

 

Response 2

The effects of K₃Fe(CN)₆ could be explained by 2 factors.

• Redox reactions of [Fe(CN)₆]^3-/[Fe (CN)₆]^4- This was confirmed by the redox peaks at 0.35 – 0.43 V and 0.18 – 0.26 V (Figure 7a), which were corresponding to the redox reactions of [Fe(CN)₆]^3-/[Fe (CN)₆]^4- couples. This could be the main reason to increase the specific capacitance.
• Facilitating the redox reactions of WO₃/SnO₂. Comparing the CV curves between with and without K₃Fe(CN)₆ (Figure 7b), the CV curves increased their area in -0.2 – 0.2 V range. For WO₃, a weak and broad reduction peak newly appeared at ~0.0 V with K₃Fe(CN)₆. For WO₃/SnO₂, the curve with K₃Fe(CN)₆ was almost parallel to that without K₃Fe(CN)₆, and a weak and broad reduction peak newly appeared at 0.0 – 0.1 V (Figure 7e). These results suggests that the enhancement in CV area was not only by the redox reaction of K₃Fe(CN)₆, but also due to the enhanced redox reaction of electrode materials.

These factors are explained in the manuscript with new references 57 and 58 as below.

LINE 415: “at 0.35 – 0.43 V and 0.18 – 0.26 V.”

LINE 437-442: “In details, weak and broad reduction bands newly appeared at -0.1 – 0.0 V for the WO₃ electrode and 0.0 – 0.1 V for WO₃/SnO₂ electrode in the presence of K₃Fe(CN)₆ (Figure 7b, c, and e). The pseudocapacitive behavior suggested that K₃Fe(CN)₆ provided the additional redox reactions of the [Fe(CN)₆]^3-/ [Fe(CN)₆]^4- couple to increase the specific capacitance of these electrodes [57, 58], and also promoted the redox reactions of the WO₃ component.”

 

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Comment 3

Why the coulombic efficiencies of WO₃/SnO₂ symmetric device is very low?

 

Response 3

The GCD curves of WO₃/SnO₂ electrode (Figure 8) were measured at the wide potential range. At the potential higher than 1.6 V (i.e. 1.8 and 1.9 V), a redox peak of water electrolysis (*corrected) appeared in the CV curve, as explained in Line 509. Although the gas generation was not obviously observed at ~1.8 V, the water hydrolysis could consume the energy. This energy loss should decrease the coulombic efficiency of WO₃/SnO₂ electrode.

Using the redox active electrolyte, the reaction products ([Fe(CN)₆]^3- and [Fe (CN)₆]^4-, mutually) diffused into the whole electrolyte solution in the cell, because the electrolyte solution was not separated into anode side and cathode side. This diffusion process also decreased the availability of [Fe(CN)₆]^3-/[Fe(CN)₆]^4- ions, as the second law of thermodynamics. For the redox-flow battery, the electrolytes in cathode and anode sides should be separated to store the energy; however, in this study, we didn’t demonstrate this application.

Thus, we added the explanation as below.

LINE 529-535: “The coulombic efficiencies were low in these cases, which could be explained by a minor water hydrolysis and the diffusion of [Fe(CN)₆]^3-/[Fe (CN)₆]^4- ions. At higher potential than 1.6 V, the edge of water electrolysis started (Figure 8a), and the minor gas generation resulted in the energy loss. Using K₃Fe(CN)₆, the reaction products ([Fe(CN)₆]^3- and [Fe(CN)₆]^4-, mutually) diffused into the whole electrolyte solution in the cell, which decreased the availability of these ions for discharge process [62].”

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Comment 4

In WO₃/SnO₂ Nanocomposite electrode, which component has a major role in obtaining high-performance symmetric supercapacitors?

 

Response 4

From Figure 5a and b, the specific capacitance of WO₃ and SnO₂ electrode were calculated to 240 F/g and 140 F/g at scan rate of 5 mV/s. This suggests that the WO₃ was more effectively store energy than SnO₂. Therefore, we consider that the WO₃ was more active to store the energy. On the other hand, the WO₃/SnO₂ nanocomposite electrode resulted in 530 F/g at the same condition (Figure 5c). This largely improved specific capacitance of WO₃/SnO₂ was due to synergistic effects in morphological and electrochemical characters. Therefore, attribution of electrical performance to specific component is difficult.

 

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Comment 5

The authors should incorporate some latest articles based on this work

 

Response 5

We added the references with a short comment as below.

LINE 56: New reference number [10]: https://doi.org/10.1016/j.cplett.2022.139884

LINE 77: New reference number [24]: https://doi.org/10.1021/acssuschemeng.1c08059

Author Response File: Author Response.docx

Reviewer 2 Report

The subject of this paper was very interesting. It was about performance of WO₃/SnO₂ nano-composite for supercapacitors. . As a result, the binder-free WO3/SnO2 nanocomposite with a redox-active electrolyte constituted a promising system for pseudocapacitive energy storage devices. But some points should be revised: 

- The order of the different sections should be corrected such as Materials and Method should be inserted after Introduction

- Authors should be added conclusion section after Discussion

 

Author Response

We deeply thank to Reviewer 2 for careful reading. We modified our manuscript according to the comments.

 

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Comments and Suggestions for Authors

The subject of this paper was very interesting. It was about performance of WO₃/SnO₂ nano-composite for supercapacitors. As a result, the binder-free WO₃/SnO₂ nanocomposite with a redox-active electrolyte constituted a promising system for pseudocapacitive energy storage devices. But some points should be revised: 

 

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Comment 1

- The order of the different sections should be corrected such as Materials and Method should be inserted after Introduction

 

Response 1

we moved the Materials and Method section after introduction.

 

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Comment 2

- Authors should be added conclusion section after Discussion

 

Response 2

We added the conclusion section after Discussion as below.

Conclusions

“In this work, we developed a preparation method of WO₃/SnO₂ nanocomposite on carbon cloth for supercapacitor electrodes. WO₃ nanospheres were first deposited on a carbon cloth substrate by a chemical bath deposition method, and then SnO₂ layer was formed by an electrochemical deposition and calcination. The WO₃/SnO₂ nanocomposite on carbon cloth exhibited higher specific capacitance (530 F/g at 5mV/s scan rate) than those of single component electrodes (240 F/g for WO₃ and 140 F/g for SnO₂) in aqueous electrolyte of 1M Na₂SO₄. The addition of 0.02 M K₃Fe(CN)₆ redox active into the 1M Na₂SO₄ electrolyte further improved the charge storage performance of electrodes (440 F/g for WO₃, 310 F/g for SnO₂, and 640 F/g for WO₃/SnO₂ nanocomposite). These results show synergistic effecs of WO₃/SnO₂ nanocomposites and advantages of using redox active electrolytes for supercapacitor systems. Using a symmetric configuration of WO₃/SnO₂ nanocomposite electrodes, the electrolyte solution of 0.02 MK₃Fe(CN)₆ and 1 M Na₂SO₄ electrolyte demonstrated a high energy density and a power density of 64 WhKg^-1 at 542 Wkg^-1, respectively. The capacitive retention efficiencies for the WO₃/SnO₂ and WO₃ electrodes with the K₃Fe(CN)₆ electrolyte was 94.7% after 2000 cycles at 2.5 Ag^-1. Thus, the WO₃/SnO₂ nanocomposite electrode grown on carbon cloth is a promising candidates as binder free electrode for high performance of supercapacitor device, and the supercapacitor system using redox active K₃Fe(CN)₆ electrolyte was proposed. These strategies developed in this study can also be applied to other metal oxide nanocomposites to improve the performance of supercapacitors.”