Investigating the Performance of Lithium-Doped Bismuth Ferrite [BiFe_1−xLi_xO₃]-Graphene Nanocomposites as Cathode Catalyst for the Improved Power Output in Microbial Fuel Cells

Round 1

Reviewer 1 Report

The manuscript presents a very detailed and diverse study of a nanocomposite as a potential cathode catalyst.

Since the reviewer is a photochemist, the remarks and questions of the report regarding mostly the photochemical parts of the manuscript.

-          In section 2.6., first UV-Vis radiation is mentioned, then only UV-radiation. Which one is right? (Within the experimental part, 3.2, the light source is not precisely given.)

-          In order to present a comparable efficiency, the quantum yield for the CIP degradation ought to be determined (or estimated, at least its minimum value, considering the scattering, too).

-          Since the band-ap energy is referred to in the case of interpretation of the results, it would be advisable to determine the CB and VB potentials with appropriate scavenging experiments.

-          Similarly, the production and role of the main oxidative agents ought to be studied by competition experiments with suitable radical scavengers.

-          The -ln(C/C_o)=kt plot should be given as referred to in the text.

Although the English of the manuscript is acceptable, it contains suspicious parts, which ought to be checked out. (Some of those are highlighted in the manuscript, along with some remarks and questions according to those above.) 

Comments for author File: Comments.pdf

Author Response

Reviewer 1

The manuscript presents a very detailed and diverse study of a nanocomposite as a potential cathode catalyst. Since the reviewer is a photochemist, the remarks and questions of the report regarding mostly the photochemical parts of the manuscript.

1. In section 2.6., first UV-Vis radiation is mentioned, then only UV-radiation. Which one is right? (Within the experimental part, 3.2, the light source is not precisely given.)

Reply: We are highly thankful to observe this error, we have corrected these typo errors in whole manuscript. The detail of the light source is also included in the experimental section 3.2.

2. In order to present a comparable efficiency, the quantum yield for the CIP degradation ought to be determined (or estimated, at least its minimum value, considering the scattering, too).

Reply: The authors are highly thankful for observing this comparable efficiency and the quantum yield. But due to some technical issue with the apparatus, we are unable to complete this task. We will definitely include this in our next article.

3. Since the band-ap energy is referred to in the case of interpretation of the results, it would be advisable to determine the CB and VB potentials with appropriate scavenging experiments.

Reply:

Figure 6 (a) UV-Visible absorption spectra of BiFe_1-xLi_xO₃-Graphene nanocomposite. (b)-(d) Tauc Plot ((αhν)² vs hν) of BiFe_1-xLi_xO₃-Graphene nanocomposite.

The UV-Visible absorption spectra of BiFe_1-xLi_xO₃-Graphene nanocomposite (x=0, 0.02, 0.4 and 0.6) samples have been shown in the Figure 6. Due to the inherent bandgap absorption, BiFe_1-xLi_xO₃-Graphene nanocomposite exhibit full-spectrum absorption across a wide range of 250-1100nm. The light absorption of the as-prepared x= 0.02 samples is dramatically increased in the region 200–800nm when compared with x=0.00 sample. The greater the absorption band of the BiFe_1-xLi_xO₃-Graphene nanocomposite, which implies that more light may be absorbed to create electron-hole pairs and subsequently enhance the photocatalytic properties of the BiFe_1-xLi_xO₃-Graphene nanocomposite. The similar type of behaviour was observed in BiFeO3/rGO nanocomposites reported by Kadi et. al.  The band gaps of BiFe_1-xLi_xO₃-Graphene nanocomposite samples was calculated by using the Tauc’s Plot ((αhν)² vs hν) displayed in the Figure 6 (b)-(d).  Bestowing to the Tauc’s Plots (ahv)²= hv-Eg, the estimated optical band gap energies (Eg) are 1.80eV, 1.7eV, 1.2eV and 1.15 eV, for x= 0.00, 0.02, 0.04 and 0.06 samples respectively. The optical band gap of pristine pure BiFeO₃ nanoparticles is 2.22eV reported in our previous papers. The addition of graphene and Li content in BiFeO₃ nanoparticles has change the band gap from 1.8eV to 1.15 eV due to intercalation of graphene sheets with the BiFeO₃ nanoparticles and change in the electron density of BiFeO₃ with the Li doping_. The analysis of absorption data acknowledged that BiFe_1-xLi_xO₃-Graphene nanocomposite have good photocatalytic performance in the presence of UV-Visible region of the electromagnetic spectrum.

Figure 8. Photocatalytic degradation of CIP in presence and absence of trapping agents (EDTA and IPA) with BiFe_1-xLi_xO₃-Graphene nanocomposite (x=0.02). (a) Absence of Trapping agents, (b) With EDTA and (c) With IPA, (d) C0/C vs time curves.

The photocatalytic activity of BiFe_1-xLi_xO₃-Graphene nanocomposite (x=0.02) for the degradation of CIP with or without trapping agents have been completed under a 110-minute exposure to UV-visible light. Figure 8 shows the photo-activity of x=0.02 photocatalysts under UV-light in the presence of trapping agents EDTA as h⁺ scavenger and IPA as .OH^- scavenger during subsequent 110 min duration (10 min intervals). The CIP concentration reduced by adding BiFe_1-xLi_xO₃-Graphene nanocomposite (x=0.02) photocatalysts with EDTA or IPA, but the rate of degradation was not same as without trapping results. Both scavenger agents capture the active species during photocatalytic reaction, as a result of this antibiotic degradation process go down. In absorption curves it shows that with addition of IPA with photocatalysts, the degradation activity slightly reduced which revels the role of .OH^- radicals is not only responsible for CIP degradation. Whereas, addition of EDTA with catalysts the photodegradation of CIP inhibited, it shows that h⁺ also participate during whole photoactivity. During trapping experiment, it seen that saturation condition also occurred after some time intervals which verify the role of O₂^- radicals in CIP degradation. The comparative kinetic graphs of x=0.02 photocatalysts for CIP degradation, without using trapping agent and in the presence of trapping agents (EDTA and IPA) verify the major role of active species in photocatalytic reaction as shown in Figure 8. The results showed that holes and superoxide oxygen radicals were mainly responsible for CIP degradation.  

4. Similarly, the production and role of the main oxidative agents ought to be studied by competition experiments with suitable radical scavengers.

Reply: We have included the included the results of the scavengers (EDTA and IPA).

5. The -ln(C/C_o)=kt plot should be given as referred to in the text.

Reply: The -ln(C/Co) vs T graph included in the inset of the Figure 7 in the revised manuscript. 

Figure 7: Photocatalytic degradation of CIP by using BiFe_1-xLi_xO₃-Graphene nanocomposites. (a) x=0.00, (b) x=0.02, (c) x=0.04 and (d) x=0.06. Inset (c) and (d) shows the C/Co Vs time and -Log(C/Co) VS Time of CIP degradation with BiFe_1-xLi_xO₃-Graphene nanocomposites.

Although the English of the manuscript is accepted, it contains suspicious parts, which ought to be checked out. (Some of those are highlighted in the manuscript, along with some remarks and questions according to those above.)

Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript focuses on the catalytic properties of graphene-supported lithium-doped bismuth ferrite (BiFe_1-xLi_xO₃) for photocatalytic oxidation of ciprofloxacin and microbial fuel cells. The catalyst characterization was well performed, while the novelty of this study needs to be thoroughly discussed, and the results and discussion sections need more explanation and discussion. For these reasons, this study is incomplete, and, unfortunately, I cannot recommend the publication of this manuscript in its present form. The authors should address the following point:

1. The UV spectra of prepared BiFe_1-xLi_xO₃ catalysts should be added.

2. In Figure 6, the values of C₀ are very different for each catalyst analyzed. It should be considered whether this will affect the comparisons.

3. The results without light irradiation should be added to Figure 6.

4. In Figure 7, the current range may need to be corrected. The current is too high for oxygen reduction in an aqueous solution system.

5. In Figure 7, is this the reduction current of oxygen? It is difficult to see the oxidation current of reductive oxygen species such as anions and anion radicals because it is unstable in aqueous solutions. As the authors claimed, if the catalytic change is reversible and the oxidation current is attributed, then much reduction current is used for that change rather than oxygen reduction. The authors should show evidence of oxygen reduction taking place.

6. The authors should characterize the catalysts before and after use.

Author Response

Reviewer 2:

This manuscript focuses on the catalytic properties of graphene-supported lithium-doped bismuth ferrite (BiFe_1-xLi_xO₃) for photocatalytic oxidation of ciprofloxacin and microbial fuel cells. The catalyst characterization was well performed, while the novelty of this study needs to be thoroughly discussed, and the results and discussion sections need more explanation and discussion. For these reasons, this study is incomplete, and, unfortunately, I cannot recommend the publication of this manuscript in its present form. The authors should address the following point:

1. The UV spectra of prepared BiFe_1-xLi_xO₃catalysts should be added.

Reply: The UV-Visible spectra of the BiFe_1-xLi_xO₃ – Gr catalysts and discussion included in the revised manuscript.

Figure 6 (a) UV-Visible absorption spectra of BiFe_1-xLi_xO₃-Graphene nanocomposite. (b)-(d) Tauc Plot ((αhν)² vs hν) of BiFe_1-xLi_xO₃-Graphene nanocomposite.

2. In Figure 6, the values of C₀are very different for each catalyst analyzed. It should be considered whether this will affect the comparisons.

Reply: Thank you for observing this in the manuscript. We have prepared 10ppm solution of the CIP freshly for each measurement and the adsorption and de-absorption of the BiFe_1-xLi_xO₃-Graphene nanocomposite samples are different. So, the Co is slightly different for each photocatalyst measurement.

3. The results without light irradiation should be added to Figure 6.

Reply: The results of the catalytic activity without light irradiation are included in the supplementary information of the article.

4. In Figure 9, the current range may need to be corrected. The current is too high for oxygen reduction in an aqueous solution system.

Reply: We would like to thank reviewers for pointing out the mistakes. Current range has been corrected and included in the supplementary information of the article.

 Figure 9: Cyclic voltammograms of [BiFe1-xLixO3] graphene composites loaded electrodes in air saturated 1M KCl solution.

5. In Figure 9, is this the reduction current of oxygen? It is difficult to see the oxidation current of reductive oxygen species such as anions and anion radicals because it is unstable in aqueous solutions. As the authors claimed, if the catalytic change is reversible and the oxidation current is attributed, then much reduction current is used for that change rather than oxygen reduction. The authors should show evidence of oxygen reduction taking place.

Reply: The reduction peak is relatively sharp and clear, while the oxidation peak is not distinct, in presence of catalyst in an oxygen saturated electrolyte. In N₂ saturated electrolyte, no reduction peak was visible (data not shown) . As rightly pointed out by the reviewers, we removed the sentences.

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6. The authors should characterize the catalysts before and after use.

Reply: The XRD and the FTIR of the catalysts (x=0.02) before and after photocatalytic activity included in the supplementary information

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Most of the answers of the authors are acceptable. The excuse regarding the lack of quantum yield determination is not really convincing.

I have found the inserted part (2.5. Optical Properties), which is OK, except for its numbering; the correct one is 2.6.

Besides, the English grammar of the manuscript has to be improved; even the newly inserted parts contain errors. 

Additionally, the authors have not corrected the lack of light source characterization in part 3.2. They have changed the power of the light source from 250 to 150w, but have not given any information on its pressure (e.g., medium-pressure or high-pressure), which could give some hints about its emission spectrum.

Hence, I think the manuscript needs a minor revision.

Author Response

Reviewer #1

1. Most of the answers of the authors are acceptable. The excuse regarding the lack of quantum yield determination is not really convincing.

Reply: The authors are highly indebted for the invaluable and crucial suggestions quoted by the reviewer. The area expertise and aptness shown by the reviewer has importantly concluded in appreciable improvement in the quality of article. We have some technical issue in our UV-Visible spectrometer. Due to this we are unable to complete this task. We will include quantum yield determination in extended part of this work.

2. I have found the inserted part (2.5. Optical Properties), which is OK, except for its numbering; the correct one is 2.6.

Reply: The authors are thankful for observing this error. We have corrected in the revised manuscript.  Page 8, line 286 annotated version.

3. Besides, the English grammar of the manuscript has to be improved; even the newly inserted parts contain errors.

Reply: We have checked the English grammar of the manuscript and done the necessary corrections in the revised manuscript. The manuscript is corrected in track change mode, so the changes can be visible throughout the manuscript.

4. Additionally, the authors has not corrected the lack of the light source characterization in part 3.2. They has changed the power of the light source from 250 to 150w, but has not given any information on its pressure (e.g., medium-pressure or high-pressure), which could give some hints about its the emission spectrum.

Reply: We have included all information of the source in the revised manuscript. The UV light source was used in this research work and the details are (Make-Lelesil Innovative Systems, India-400604, 150 W UV Range: Medium Pressure lamp with built in resister and quartz glass outer sell),  Page 18, line 604-605 annotated version.

5. Hence, I think the manuscript needs a minor revision.

Reply: Thank you sir. The changes done in the manuscript are highlighted in colour font.

After such changes in the manuscript, we hope you will find the paper suitable for publication in Catalysts.

Reviewer 2 Report

The authors have revised properly the manuscript based on the reviewer’s comments. I think that the manuscript in the present form could be accepted for publishing.

Author Response

The authors have revised properly the manuscript based on the reviewer’s comments. I think that the manuscript in the present form could be accepted for publishing.

Reply: Thank you Sir/ma’am.