A Bifunctional Electroactive Ti₄O₇-Based Membrane System for Highly Efficient Ammonia Decontamination

Round 1

Reviewer 1 Report

Dear Authors,

Below you can find my opinion about your submitted manuscript entitled „ A bifunctional Electroactive Ti4O7-based Membrane System for Highly Efficient Ammonia Decontamination” in MDPI „Catalysts” Journal.

After careful reading of your work, I would like to share with you my suggestions, which in my opinion are missing for me – as a reader. Moreover, I would like to underline the fact, the presented research is highly innovative and can be published after major revision of the manuscript.
My suggestions are as follow:

Page 2/11 – line 80-82.

1) „This work is dedicated to providing an effective and affordable design…”
There is any graphs/plots/scheme of the described setup. I understand the fact this flow-system has been presented elsewhere but based on previous statement authors underlined that the implementation of the flow is the most essential and crucial part of work. The graph or even idea-scheme have to be implemented into the revised version of the manuscript.

2) Full characterization of the working electrode in your system is a must.
On presented research, authors are using the electrochemical method to decontamination of the ammonia in aqueous solutions. The information about the electrochemically active surface area of electrodes should be delivered. What is the “real” electrochemical active surface area of the anode, cathode (Ni foam and Ni-decorated Cu/Pd electrode.) How the system is working in case of “flat planar electrodes and the case of “highly developed surface area. If differences between these (Ni and Ni/CuPd) cathode active surface is significant, this type of comparison can be not accurate.

3) Ammonia conversion

Authors presented many results connected with the conversion rate of ammonia and distribution of different decontamination products which is an essential part of submitted work. I would like to see real measured changes of ammonia concentration which was estimated by UV-vis method. Presentation of the directly obtained results increases the level of understanding of how the authors were measured the initial and final ammonia concentration. A similar case can be applied to the NO3- and NO2- species in the electrolyte.

4) Condition of the flow in the system
What about the hydrodynamic condition in the flow? How the system behaves in a variation of the flow rate? This information could be very valuable for the readers. Fig 3,4 – flow rate is 1.5 ml/min. What’s about the other presented results?

5) The visual part of the work
All of the presented results are nice and easy to understand. I would ask you to modify the figure S5 due to non-significant differences between the presented series of experiments, and they are overlapping.

With my best regards,

 

Author Response

Comments by Reviewer #1:  

Dear Authors, Below you can find my opinion about your submitted manuscript entitled „ A bifunctional Electroactive Ti4O7-based Membrane System for Highly Efficient Ammonia Decontamination” in MDPI „Catalysts” Journal.

After careful reading of your work, I would like to share with you my suggestions, which in my opinion are missing for me – as a reader. Moreover, I would like to underline the fact, the presented research is highly innovative and can be published after major revision of the manuscript.

Reply: We appreciate the reviewer’s positive comments. We have carried out the following point-by-point revisions based on the reviewer’s comments and/or suggestions.

 

My suggestions are as follow:

Page 2/11 – line 80-82.

1) „This work is dedicated to providing an effective and affordable design…”

There is any graphs/plots/scheme of the described setup. I understand the fact this flow-system has been presented elsewhere but based on previous statement authors underlined that the implementation of the flow is the most essential and crucial part of work. The graph or even idea-scheme have to be implemented into the revised version of the manuscript.

Reply: We have provided digital pictures and schematic illustrations of the experimental setup.

 

Revision:

Line 262,

 

“The ammonia degradation experiment was carried out in a customized flow-through electrochemical apparatus (Figure S8), including a Ti₄O₇ working electrode (D = 12 mm), a Pd-Cu/NF counter electrode (D = 12 mm), and a saturated Ag/AgCl reference electrode.”

Figure S8. Schematic of the flow-through filtration setup. (a) Modified acrylic filter sleeve, including 1) a Ti₄O₇ anode, 2) a Ti plate current collector, and 3) a Pd-Cu/NF cathode. (b, d) Images of the modified filter sleeve. (c, e) Images of the anode and cathode.

 

2) Full characterization of the working electrode in your system is a must.

On presented research, authors are using the electrochemical method to decontamination of the ammonia in aqueous solutions. The information about the electrochemically active surface area of electrodes should be delivered. What is the “real” electrochemical active surface area of the anode, cathode (Ni foam and Ni-decorated Cu/Pd electrode.) How the system is working in case of “flat planar electrodes and the case of “highly developed surface area. If differences between these (Ni and Ni/CuPd) cathode active surface is significant, this type of comparison can be not accurate.

Reply: (1) The electrochemical filtration experiments were performed in a customized apparatus. Both anode and cathode are in circular form with diameter of 12 mm; (2) when performing ammonia oxidation using flat planar electrode, the solution only flow-by the electrode surface in a conventional batch reactor. We have clarified this information in the new version.

 

 

Revision:

Line 117,

 

“……While the removal kinetics was the lowest in single-pass filtration mode, because the liquid residence time within the filtration device was rather limited (e.g., 5 sec), which caused insufficient contact between ammonia molecules and the electrode. In addition, in the batch mode, the mass transport of ammonia was only dominated by diffusion and, consequently, only the surface active sites contributed to the ammonia conversion. This also demonstrates the advantage of a flow-through design compared to the flow-by mode in conventional batch system.”

 

Line 262,

 

“The ammonia degradation experiment was carried out in a customized flow-through electrochemical apparatus (Figure S8), including a Ti₄O₇ working electrode (D = 12 mm), a Pd-Cu/NF counter electrode (D = 12 mm), and a saturated Ag/AgCl reference electrode.” 

 

3) Ammonia conversion

Authors presented many results connected with the conversion rate of ammonia and distribution of different decontamination products which is an essential part of submitted work. I would like to see real measured changes of ammonia concentration which was estimated by UV-vis method. Presentation of the directly obtained results increases the level of understanding of how the authors were measured the initial and final ammonia concentration. A similar case can be applied to the NO3- and NO2- species in the electrolyte.

Reply: Sorry for the misunderstanding caused. Both influent and effluent ammonia concentration were determined by UV-vis method while passing through the electrochemical filtration system. both NO₃^- and NO₂^- were also determined by the IC method. We have clarified this in the new version.

 

Revision:

Line 268,

 

“……Ammonia concentration in the influent and effluent were measured by the standard Nessler reagent method. NO₃^- and NO₂^- concentration was determined by a Thermo Fisher DionexTM ICS-5000 ion chromatography (IC, USA).”

 

Line 112,

 

“The catalytic performances of the Ti₄O₇ electrodes were evaluated using the ammonia oxidation as a model reaction. As shown in Figure S5, an evident promotion was observed for ammonia conversion in the flow-through system with effluent ammonia concentration decreased from 30 to 0.7 mg·L^-1. However, for the batch and single-pass system, the ammonia concentration decreased from 30 to 15.1 mg·L^-1 and 26.3 mg·L^-1, respectively, under similar conditions.”

Figure S5. Comparison of ammonia conversion kinetics in batch, single-pass and flow-through systems. Reaction conditions: anode potential of 2.8 V vs. Ag/AgCl, [Cl^-] concentration of 0.14 M, and pH of 9.

 

Line 215,

 

“……Results suggest that a similar ammonia removal kinetics were observed using a Cu-NF or a Pd-NF cathode, and the NO₃^- generated was 15.9 and 17.1 mg·L^-1, respectively. However, the integration of Pd-Cu sites evidently boosted the kinetics of NO₃^- reduction to N₂ and inhibits the conversion of NH₄⁺ to NO₃^-, resulting in only 5.1 mg·L^-1 NO₃^-, while the presence of Pd or Cu-alone only lead to limited N₂ yield (Figure 3)..…”

Figure 3. The conversion of ammonia in 90 min by employing different cathode materials: (a) Pd-Cu/NF, (b) Pd/NF, (c) Cu/NF, and (d) NF. Reaction conditions: anode potential of 2.8 V vs. Ag/AgCl, [Cl^-] of 0.14 M, flow rate of 1.5 mL·min^-1, and pH 9.

 

4) Condition of the flow in the system

What about the hydrodynamic condition in the flow? How the system behaves in a variation of the flow rate? This information could be very valuable for the readers. Fig 3,4 – flow rate is 1.5 ml/min. What’s about the other presented results?

Reply: Thank for the good suggestion. We have supplemented the impact of flow rate on the ammonia conversion performance. 

 

Revision:

Line 162,

 

“2.3.4. Effect of flow rate

The effect of flow rate on the ammonia oxidation were available in Figure S6. when the flow rate increased from 0.5 to 1.5 mL·min^-1, the ammonia conversion efficiency increased from 73.7% to 97.4% consequently. However, further increasing flow rate to 2.0 mL·min^-1 caused a slight decrease in ammonia oxidation performance. This can be explained by the fact that a higher flow rate may shorten residence time and cause incomplete ammonia reaction.”

Figure S6. The effect of flow rate on ammonia conversion. Reaction conditions: anode potential of 2.8 V vs. Ag/AgCl, [Cl^-] concentration of 0.14 M, and pH of 9.

 

5) The visual part of the work

All of the presented results are nice and easy to understand. I would ask you to modify the figure S5 due to non-significant differences between the presented series of experiments, and they are overlapping.

Reply: Thank you for the good suggestion. We have re-prepared this figure for better presentation. 

 

Revision:

Page S9, Supporting Information,

 

Figure S7. Conversion effect of ammonia with 0.14 M Na₂SO₄ background electrolyte at different anode potentials. Experimental conditions: [Cl^-] of 0.14 M, flow rate of 1.5 mL·min^-1, and pH of 9.

Author Response File: Author Response.pdf

Reviewer 2 Report

Authors reported an electro active filtration system to decontaminate ammonia from water. They applied convection-enhanced mass transport for enhanced kinetics with stability and environmental matrixes. In the section of characterization, XPS analysis has to be revised as their deconvolution process didn't follow the spin-orbit splitting rule exactly. In addition it is recommended to build a table for comparison with more reference literatures. Comments and questions are the following.

1. Figure S1(a): The main characteristic peaks of Magnéli phase _Ti4O7 have to be mentioned in the main text and labeled in the figure. 

2. Figure S1(c): The region below 200 eV isn't shown. It is recommended to display the full range of survey spectrum for checking any further impurity peak. In addition, O 1s and C 1s spectra are required for confirming the stoichometry of Ti₄O₇ and any carbonaceous impurity. 

3. Figure S1(d): The orange peak labeled for Ti⁴⁺ at 456.2 eV is assigned for Ti³⁺. It seems to be a typo. Ref. [16] may be ok for Ti⁴⁺, not Ti³⁺. In the Figure 5(a) and c(c) of Ref. [16], they assigned a small peak at 456.7 eV for Ti-S without having its spin-orbit splitting 2p_1/2 peak. Another reference is required for supporting Ti³⁺.

4. Figure S3(c): Spin-orbit splitting has the peak area ratios. In case of p orbital, it is 1:2 for 2p_1/2:2p_3/2. This ratio has to be kept in the deconvolution process, or the peak positions could be changed for wrong analysis. 

5. Figure S3(d): Cu 2p spectrum is seen for only 2p_3/2 peaks. It is recommended to display the whole range from 930 to 970 eV, because 2p_1/2 peaks are also helpful for distinguishing various Cu species.

6. Figure S4: Only one condition isn't enough for the demonstration of enhanced performance, compared to others. It is suggested to test the ammonia oxidation with batch and single-pass system under all the same conditions in Figure 1. Actually 2.8 V, 0.14 M and pH 9 are optimized for flow-through method only, not others. It may mean they have different conditions for their best performance. They may show superior results then flow-through methods at a certain condition. 

Author Response

Comments by Reviewer #2:

Authors reported an electro active filtration system to decontaminate ammonia from water. They applied convection-enhanced mass transport for enhanced kinetics with stability and environmental matrixes. In the section of characterization, XPS analysis has to be revised as their deconvolution process didn't follow the spin-orbit splitting rule exactly. In addition it is recommended to build a table for comparison with more reference literatures. Comments and questions are the following.

Reply: We appreciate the reviewer’s positive comments. We have carried out the following point-by-point revisions based on the reviewer’s comments and/or suggestions. We also supplemented a table for comparison with references on the electrochemical ammonia oxidation.

 

Page S2, Supporting Information,

Table S1. Summary on the electrochemical ammonia conversion using different electrode materials.  

Anode	Ammonia concentration	Experimental conditions	Removal efficiency	References
WO3	20 mg·L-1	Cl-/NH4+ = 20, pH = 4, 1.0 V vs. SCE	99.9% (1.5 h)	[1]
SnO2-CNT	30 mg·L-1	2.5 V vs. Ag/AgCl, [Cl-] = 0.1 M, pH = 7 flow rate = 4 mL·min-1	＞99.9% (1.5 h)	[2]
Ti/Pt	150 mg·L-1	NaCl = 0.8% (w/v) pH = 9 I = 0.075 A·cm-2	82% (1 h)	[3]
Ti/RuO2-TiO2	1060-1380 mg·L-1	I = 116 mA·cm-2 flow rate = 2000 L·h-1	49% (3 h)	[4]
Ti4O7	30 mg·L-1	2.8 V vs. Ag/AgCl, [Cl-] = 0.14 M, pH = 7 flow rate = 1.5 mL·min-1	100% (40 min)	This study

 

[1] Ji Y.Z.; Bai J.; Li J.H.; Luo T.; Qiao L.; Zeng Q.Y.; Zhou B.X. Highly selective transformation of ammonia nitrogen to N₂ based on a novel solar-driven photoelectrocatalytic-chlorine radical reactions system. Water Res. 2017, 125, 512-519.

[2] Li F.; Peng X.; Liu Y.B.; Mei J.C.; Sun L.W.; Shen C.S.; Ma C.Y.; Huang M.H.; Wang Z.W.; Sand W.G. A chloride-radical-mediated electrochemical filtration system for rapid and effective transformation of ammonia to nitrogen. Chemosphere 2019, 229, 383-391.

[3] Vlyssides A.G.; Karlis P.K.; Rori N.; Zorpas A.A. Electrochemical treatment in relation to pH of domestic wastewater using Ti/Pt electrodes. J. Hazard. Mater. 2002, 95, 215-226.

[4] Moraes P.B.; Bertazzoli R. Electrodegradation of landfill leachate in a flow electrochemical reactor. Chemosphere 2005, 58, 41-46.

 

1. Figure S1(a): The main characteristic peaks of Magnéli phase Ti4O7 have to be mentioned in the main text and labeled in the figure.

Reply: We have revised the Figure S1a and mentioned the characteristic peaks of Magnéli phase Ti₄O₇.

 

Revision:

Line 87,

 

“X-ray diffraction (XRD) patterns of the electrode materials in well accordance with that of standard spectra of Ti₄O₇, with characteristic peaks centered at 20.8°, 26.2°, 29.5°, 31.6°, 34.0°, 36.2°, 40.5°, 53.1°, 55.0°, 63.7°, and 66.3° (indicated by red points) (Figure S1b)……”

Figure S1. (a) FESEM characterization, (b) XRD pattern, and (c) XPS survey pattern of the Ti₄O₇ anode. XPS narrow scan of (d) Ti 2p on Ti₄O₇ electrode.

 

2. Figure S1(c): The region below 200 eV isn't shown. It is recommended to display the full range of survey spectrum for checking any further impurity peak. In addition, O 1s and C 1s spectra are required for confirming the stoichometry of Ti4O7 and any carbonaceous impurity.

Reply: We have revised Figure S1c to display the full range of the survey pattern. We also clarified the O 1s and C 1s spectra in the new version. Thank you for the good suggestion!

 

Revision:

Line 98,

 

“……As displayed in Figure S2, the C 1s XPS spectra can be divided into three peaks centered at 284.5 eV, 285.9 eV, and 288.3 eV, corresponds to C=C, C-O and C=O, respectively. While the high resolution O 1s spectrum were deconvoluted into two peaks centered at 529.7 eV and 531.1 eV, respectively, corresponds to Ti-O bond and -OH group [18]…….”

Figure S2. XPS spectra for the narrow scan of (c) C 1s, and (d) O 1s on Ti₄O₇ electrode.

 

Page S3, Supporting Information,

Figure S1. (a) FESEM characterization, (b) XRD pattern, and (c) XPS survey pattern of the Ti₄O₇ anode. XPS narrow scan of (d) Ti 2p on Ti₄O₇ electrode.

 

3. Figure S1(d): The orange peak labeled for Ti4+ at 456.2 eV is assigned for Ti3+. It seems to be a typo. Ref. [16] may be ok for Ti4+, not Ti3+. In the Figure 5(a) and c(c) of Ref. [16], they assigned a small peak at 456.7 eV for Ti-S without having its spin-orbit splitting 2p1/2 peak. Another reference is required for supporting Ti3+.

Reply: Thank you for the good suggestion. We have corrected this mistake and provided additional reference for supporting Ti³⁺.

 

Revision:

Line 94,

 

“X-ray photoelectron spectroscopy (XPS) survey pattern of the Ti₄O₇ electrode confirms the presence of C 1s, O 1s, and Ti 2p with specific ratio of 21.91, 54.92 and 23.17%, respectively. The Ti 2p characteristic peaks centered at 458.7 eV (2p_3/2) and 464.3 eV (2p_1/2) can be assigned to Ti⁴⁺. While these peaks located at 456.2 eV (2p_3/2) and 461.9 eV (2p_1/2) can be assigned to Ti³⁺ (Figure S1d) [16, 17].”

 

[16] Tao X.Y.; Wang J.G.; Ying Z.G.; Cai Q.X.; Zheng G.Y.; Gan Y.P.; Huang H.; Xia Y.; Liang C.; Zhang W.K.; Cui Y. Strong sulfur binding with conducting Magneli-phase Ti_nO_2n-1 nanomaterials for improving lithium-sulfur batteries. Nano Lett. 2014, 14, 5288-5294.

[17] Boffa A.B.; Galloway H.C.; Jacobs P.W.; Benitez J.J.; Batteas J.D.; Salmeron M.; Bell A.T.; Somorjai G.A. The growth and structure of titanium oxide films on Pt(111) investIgated by LEED, XPS, ISS, and STM. Surf. Sci. 1995, 326, 80-92.

 

4. Figure S3(c): Spin-orbit splitting has the peak area ratios. In case of p orbital, it is 1:2 for 2p1/2:2p3/2. This ratio has to be kept in the deconvolution process, or the peak positions could be changed for wrong analysis.

Reply: We have revised accordingly. Thank you!

 

 

 

Revision:

Page S6, Supporting Information,

 

Figure S4. FESEM images (a, b) of the Pd/Cu-Ni foam electrode. XPS spectra for the narrow scan of (c) Pd, and (d) Cu of a fresh Pd−Cu/NF cathode.

 

5. Figure S3(d): Cu 2p spectrum is seen for only 2p3/2 peaks. It is recommended to display the whole range from 930 to 970 eV, because 2p1/2 peaks are also helpful for distinguishing various Cu species.

Reply: We have revised accordingly.

 

Revision:

Line 109,

 

“……Due to the presence of Cu⁰, Cu⁺, and Cu²⁺, the Cu 2p_3/2 spectra can be deconvoluted into three peaks centered at 931.8, 934.4, and 942.7 eV, respectively. The Cu 2p_1/2 characteristic peaks centered at 953.0 and 962.5 eVwere assigned to Cu²⁺ (Figure S4d) [6, 19].”

 

[6] Li F.; Peng X.; Liu Y.B.; Mei J.C.; Sun L.W.; Shen C.S.; Ma C.Y.; Huang M.H.; Wang Z.W.; Sand W.G. A chloride-radical-mediated electrochemical filtration system for rapid and effective transformation of ammonia to nitrogen. Chemosphere 2019, 229, 383-391.

[19] Liu C.; Hirohara M.; Maekawa T.; Chang R.; Hayashi T.; Chiang C.Y. Selective electro-oxidation of glycerol to dihydroxyacetone by a non-precious electrocatalyst - CuO. Appl. Catal. B-Environ. 2020, 265, 12.

 

6. Figure S4: Only one condition isn't enough for the demonstration of enhanced performance, compared to others. It is suggested to test the ammonia oxidation with batch and single-pass system under all the same conditions in Figure 1. Actually 2.8 V, 0.14 M and pH 9 are optimized for flow-through method only, not others. It may mean they have different conditions for their best performance. They may show superior results then flow-through methods at a certain condition.

Reply: Thank you for the good suggestion. For an electrochemical system, the oxidation capability was dominantly determined by the electrode materials, especially the anode materials. In our study, we employed the same anode and cathode materials for all three operational modes (i.e., recirculation, single-pass, and batch), we, thus, believed that the oxidation capability and optimal conditions were similar for all cases. The only difference was originated from the mass transport. Therefore, we only choose the same experimental conditions to compare with other two modes. 

 

Revision:

Line 113,

 

“The catalytic performances of the Ti₄O₇ electrodes were evaluated using the ammonia oxidation as a model reaction. As shown in Figure S5, an evident promotion was observed for ammonia conversion in the flow-through system with effluent ammonia concentration decreased from 30 to 0.7 mg·L^-1. However, for the batch and single-pass system, ammonia decreased from 30 to 15.1 mg·L^-1 and 26.3 mg·L^-1, respectively, under similar conditions……”

Figure S5. Comparison of ammonia conversion kinetics in batch, single-pass and flow-through systems. Reaction conditions: anode potential of 2.8 V vs. Ag/AgCl, [Cl^-] concentration of 0.14 M, and pH of 9.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Dear Authors, 

I am very glad to see the fact, all of the suggestions have been implemented into the revised version of the manuscript. 

Unfortunately, there is one issue which has to be addressed and corrected in manuscript. The surface of the working electrode is an essential factor in all catalytic processes. I suppose my question was not clearly explained, and obtained answer was not what I expected. 

 

The electrochemical active surface area (ECSA) is describing the REAL surface of the electrode. On the manuscript, authors claim that the catalytic processes take place on the "active sites" on the material. How are you measure it? For explanation: If you have ideally polished substrate, and you modify it by atomic layer deposition (For ex. Cu-Pd alloy deposition), the electrochemically active area can be described by the geometric area of the sample. 

If you are doing the deposition process on the non-polished titanium plate by conventional electrodeposition, the electrochemically active surface area will be bigger than this from the previous case. But still, for some electrochemical investigations, this is accurate enough. 

In your case, the catalyst is deposited on porous foam structure - it means the 1 cm2 of your material has many times bigger the real electrochemical surface area. I assume the "bare" substrate has uniform structure, size of the pores and density which allows comparing the CuPd foam with the reference sample. Based on these facts, the information about the real active electrode surface in your case is critical for the comparison with other materials tested on porous substrates. 

The real electrochemical surface area can be easily determined by various methods (hydrogen underpotential deposition (HUPD), BET (N2 adsorption), double layer capacity measurements.)

Electrochemical determination of the ECSA are very well-described on paper : J. Am. Chem. Soc. 2013, 135, 45, 16977-16987 (10.1021/ja407115p) and can be useful for your study. After these changes, I will be glad to accept this article for publication.

With my best wishes,

 

 

Author Response

Comments by Reviewer #1:  

I am very glad to see the fact, all of the suggestions have been implemented into the revised version of the manuscript.

 

Unfortunately, there is one issue which has to be addressed and corrected in manuscript. The surface of the working electrode is an essential factor in all catalytic processes. I suppose my question was not clearly explained, and obtained answer was not what I expected.

 

The electrochemical active surface area (ECSA) is describing the REAL surface of the electrode. On the manuscript, authors claim that the catalytic processes take place on the "active sites" on the material. How are you measure it? For explanation: If you have ideally polished substrate, and you modify it by atomic layer deposition (For ex. Cu-Pd alloy deposition), the electrochemically active area can be described by the geometric area of the sample.

 

If you are doing the deposition process on the non-polished titanium plate by conventional electrodeposition, the electrochemically active surface area will be bigger than this from the previous case. But still, for some electrochemical investigations, this is accurate enough.

 

In your case, the catalyst is deposited on porous foam structure - it means the 1 cm2 of your material has many times bigger the real electrochemical surface area. I assume the "bare" substrate has uniform structure, size of the pores and density which allows comparing the CuPd foam with the reference sample. Based on these facts, the information about the real active electrode surface in your case is critical for the comparison with other materials tested on porous substrates.

 

The real electrochemical surface area can be easily determined by various methods (hydrogen underpotential deposition (HUPD), BET (N2 adsorption), double layer capacity measurements.)

 

Electrochemical determination of the ECSA are very well-described on paper : J. Am. Chem. Soc. 2013, 135, 45, 16977-16987 (10.1021/ja407115p) and can be useful for your study. After these changes, I will be glad to accept this article for publication.

Reply: We really appreciate the reviewer for his kind and detailed explanation. We have provided the BET data for anode and cathode materials in the new version. However, due to the Coronavirus, our university was closed for the time being and we cannot supplement other electrochemical determinations. Hope you can understand.

 

Revision:

Line 84,

 

“The morphology of Ti₄O₇ electrodes were characterized by field emission scanning electron microscopy (FESEM). Figure S1a reveals that the macroscale T_i4O₇ particles were interconnected and formed macroporous structure. This lead to a BET surface area of 2.8±0.4 m² g^-1 for the Ti₄O₇ anode [14].”

 

Line 102,

 

“Compared to original gray NF, the color of the Pd-Cu/NF changed to black after the loading of Cu and Pd (Figure S3, although the macropore size and surface morphology did not change significantly (Figure S4) [6]. Such macroporous dimension of the Pd-Cu/NF cathode only generated a BET surface area of 1.3±0.1 m² g^-1.”  

 

Line 282,

 

“The Brunauer Emmett Teller (BET) surface area of the electrode materials were performed using an Autosorb iQ-c analyzer (Quantachrome, USA).”

Author Response File: Author Response.pdf

Reviewer 2 Report

Authors revised the manuscript well according to the comments and questions with additional table and updated figures as well as proper reference literatures, except Figure S4(d). They assigned three copper oxidation states: Cu⁰, Cu¹⁺, and Cu²⁺ at 931.8, 934.4 and 942.7 eV, respectively. In addition both peaks at 953.0 and 962.5 eV are for Cu²⁺ 2p_1/2. If so, it doesn't make sense, because both Cu⁰ and Cu¹⁺ also need the spin-orbit splitting peaks for Cu 2p_1/2. This has to be corrected prior to publication. 

Author Response

Comments by Reviewer #2:

Authors revised the manuscript well according to the comments and questions with additional table and updated figures as well as proper reference literatures, except Figure S4(d). They assigned three copper oxidation states: Cu0, Cu1+, and Cu2+ at 931.8, 934.4 and 942.7 eV, respectively. In addition both peaks at 953.0 and 962.5 eV are for Cu2+ 2p1/2. If so, it doesn't make sense, because both Cu0 and Cu1+ also need the spin-orbit splitting peaks for Cu 2p1/2. This has to be corrected prior to publication.

Reply: Thank you for pointing out our mistake. We have corrected and rewritten this part. Thank you!  

 

Revision:

Line 108,

 

“The successful deposition of Cu onto the NF can be verified by the Cu 2p XPS spectra. As displayed in Figure S4d, the signals located at 931.8 eV and 953.0 eV were assignable to the emissions from Cu 2p 3/2 and Cu2p 1/2 levels, which corresponds to the typical binding energy of Cu⁰ [19]. Other binding energies present at 934.4 eV for Cu 2p_3/2 and 954.0 eV for Cu 2p_1/2 were ascribed to to Cu²⁺ [6, 20]. Besides these peaks, the presence of shake-up satellite peaks within the region of 940–945 eV (Cu 2p_3/2) and 959-965 eV (Cu 2p_1/2) also indicated the presence of Cu²⁺ species on the cathode surface. Similar phenomenon was also previously documented [21].”

 

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

Dear Authors, 

Thank you for adding the BET analysis for anode and cathode materials. 

I am fully satisfied with all your reply and corrections connected to the manuscript. I would inform the editor about my positive feedback and ask for the acceptation for publication.

 

Best regards,