A Paper-Based Microfluidic Fuel Cell Using Soft Drinks as a Renewable Energy Source

Round 1

Reviewer 1 Report

Summary:

This work titled “A Paper-based Microfluidic Fuel Cell Using Soft Drinks as a Renewable Energy Source” reports a very minty result with Coca Cola, Pepsi, and 7 up to construct an air-breathing paper-based microfluidic fuel cell with common soft drinks. The concept is analyzed by electrochemical measurements and half cells. The results and analysis suggest the possibility to build microfluidic fuel cell with soft drinks as the fuel.

 

Comments:

1. Why Dr. Pepper shows a relatively unstable polarization and power density curves?

 

2. Why the cold soft drinks have a relatively poor stability in the polarization and power density curves?

Author Response

Dear Reviewer

 

The authors appreciate the corrections and hope that the answers enlisted below in this document fulfill the expectations. We considered every suggestion as a valuable opportunity to improve and enrich our work enormously.

Therefore, we have intended to be as punctual as we could to attend the decision of Major Revisions. Reviewer observations are highlighted using bold black font, our replies using a blue font and changes in existing sentences have been written in italic font.

 

Reviewer

 

This work titled “A Paper-based Microfluidic Fuel Cell Using Soft Drinks as a Renewable Energy Source” reports a very minty result with Coca–Cola^®, Pepsi^®, Dr. Pepper^®, and 7up^® up to construct an air-breathing paper-based microfluidic fuel cell with common soft drinks. The concept is analyzed by electrochemical measurements and half cells. The results and analysis suggest the possibility to build microfluidic fuel cell with soft drinks as the fuel.

 

Comments:

1. - Why Dr. Pepper shows a relatively unstable polarization and power density curves?

Thank you very much for the comments. Please, could you consider, section 2.4, line 125, and describe: “In these experiments, the soft drinks were neither deoxygenated nor degassed.” In this sense, the soft drink has CO₂, this CO₂ present water solubility, but the solubility is better in cold temperatures and is poorer when the temperature increment. Then, the experiment could have a bubble in the catalytic surface, causing an area in this surface without fuel, so the current and voltage have a variation. This variation is the cause of unstable in the polarization curve; this phenomenon is present, with major possibility when having a soft drink at room temperature.

 Additionally, it is an uncontrolled experimental condition, and the stability can occur in Coca–Cola®, and Dr. Pepper^®, or not present as in the case of 7up^® and Pepsi^®. These instabilities can be smoothed; however, it was decided not to smooth, to have results with the most near to reality.

 

Now we have added this explanation in the text (Section 2.4-Line 125): “This condition could cause a bubble in the catalytic surface, causing an area in this surface without fuel, so the current and voltage could have a variation. This variation could cause unstable in the polarization curve.”

 

2. - Why the cold soft drinks have a relatively poor stability in the polarization and power density curves?

Thank you very much for your observations.

We believe that your comment refers to the polarization and power density curves presented in Figure 6 (page 7), and specifically to the experimentation using the soft drinks at room temperature.

In this sense, the reason for the instability is described in section 3.2, “Soft drinks paper-based microfluidic fuel cell performance” between the 171 and 185 lines.

We explained that a decrease in temperature would cause an increase in the  factor (which is part of the Butler-Volmer equation), considering that the other parameters (of the Butler-Volmer equation) do not change because the same surface is used as a catalyst and the same medium, increasing the contribution of the oxygen reduction reaction current and, then, the performance of the cell.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript describes an evaluation of microfluidic fuel cells using carbohydrate (mainly glucose) from soft drinks as fuel. It will benefit from revision for accuracy and clarity. For example, "... zones in ..." should be "... zones in the presence of ..."; "originated" should be "stimulated"; "... they are disposable fuel cells' should be "... the fuel cells are disposable"; "The authors, in their best performance ..." should be "In the best performance, the cell ..." (It isn't the performance of the authors that is being described); "The authors described the system ..." would better be "The system was described ..."; "In this paper, the energy was ..." should be "soft drinks have been used as an energy source ..." (Energy was not generated in the paper but rather in the cell - results are presented in the paper). These represent examples of the kinds of corrections needed throughout.

Author's names, personal pronouns and et. al. should be omitted. 

Author Response

Dear Reviewer

 

The authors appreciate the corrections and hope that the answers enlisted below in this document fulfill the expectations. We considered every suggestion as a valuable opportunity to improve and enrich our work enormously.

 

Therefore, we have intended to be as punctual as we could to attend the decision of Major Revisions. Reviewer observations are highlighted using bold black font, our replies using a blue font and changes in existing sentences have been written in italic font.

 

Reviewer

 

This manuscript describes an evaluation of microfluidic fuel cells using carbohydrate (mainly glucose) from soft drinks as fuel. It will benefit from revision for accuracy and clarity. For example, "... zones in ..." should be "... zones in the presence of ..."; "originated" should be "stimulated"; "... they are disposable fuel cells' should be "... the fuel cells are disposable"; "The authors, in their best performance ..." should be "In the best performance, the cell ..." (It isn't the performance of the authors that is being described); "The authors described the system ..." would better be "The system was described ..."; "In this paper, the energy was ..." should be "soft drinks have been used as an energy source ..." (Energy was not generated in the paper but rather in the cell - results are presented in the paper). These represent examples of the kinds of corrections needed throughout.

Author's names, personal pronouns and et. al. should be omitted.

 

 

Line 4, made the change: “zones in” by “zones in the presence of”, and the same to lines; 164, 176, and 246

 

Line 19, and 126 made the change: “originated” by “stimulated”

 

 Line 33, made the change: “they are disposable-fuel cells” by “the fuel cells are disposable”.

 

Line 50, made the change; “The authors, in their best performance” by “The µFC in the best performance”

 

Line 56, made the change: “The authors described the systems” by “The system was described”

 

Line 52, made the change: “The authors” by “The present investigation”

 

Line 240, made the change: “the energy was obtained using soft drinks as an energy source” by “the soft drinks have been used as an energy source”

 

Line 48 made the change: “Dan Wen et al.” by “Research has”

 

Line 54 made the change: “You  Yu  et al.” by “research”

Author Response File: Author Response.pdf

Reviewer 3 Report

In this paper, the author discussed about the fabrication of paper based microfluidic fuel cell using some soft drinks. I recommend to accept after a minor revision.

 

1. Already, a lot research works are published in fabrication of paper based microfluidic fuel cell. Therefore, author should explain about the novelty of this approach.
2. The author should explain about the selectivity towards use of soft drinks in fuel cell application
3. Soft drinks contains lot of gas molecule components, and it will produce peroxides easily. Author should explain about this issue.

Author Response

Dear Reviewer

 

The authors appreciate the corrections and hope that the answers enlisted below in this document fulfill the expectations. We considered every suggestion as a valuable opportunity to improve and enrich our work enormously.

 

Therefore, we have intended to be as punctual as we could to attend the decision of  Revisions. Reviewer observations are highlighted using bold black font, our replies using a blue font and changes in existing sentences have been written in italic font.

 

Reviewer

 

In this paper, the author discussed about the fabrication of paper based microfluidic fuel cell using some soft drinks. I recommend to accept after a minor revision.

 

1. Already, a lot research works are published in fabrication of paper based microfluidic fuel cell. Therefore, author should explain about the novelty of this approach.

 

Now we have added this explanation in the text:

Section 1, line 76

“The novelty of this work is the use of inorganic catalyst material in paper-based µFC for power generation using soft drinks as fuels. This approach has not been widely studied in this type of material. Also, a new concept of emergency energy through this paper-based µFC was created. Furthermore, due to the easy access to sugary soft drinks that we have today, these paper microfluidic fuel cells could actually be applied as possible backup energy sources for low-consumption electronic devices.”

 

2. The author should explain about the selectivity towards use of soft drinks in fuel cell application.

 

Dear reviewer, we attach the following ideas in section 2.4 line 130, where we write the application of our fuel cell that uses soft drinks:

 

“Even though the soft drinks are available at a low cost in large quantities, the paper-based µFC present in this work is intended to be fed with a drop of soft drinks from the straw or with a small amount dropped of soft drink while the µFC remains connected to some device, which limits the available volume of fuel significantly.”

 

3. Soft drinks contains lot of gas molecule components, and it will produce peroxides easily. Author should explain about this issue.

 

Carbonated soft drinks contain a lot of dissolved gases, mostly CO₂, which is a molecule that has a high amount of resonant structures, which makes it a chemically very stable molecule, which causes it to be a little reactive substance. Therefore, the generation of H₂O₂ from the gases dissolved in these soft drinks is deficient.

Author Response File: Author Response.pdf

Reviewer 4 Report

Please see the report attached for details

Comments for author File: Comments.pdf

Author Response

Dear Reviewer

 

The authors appreciate the corrections and hope that the answers enlisted below in this document fulfill the expectations. We considered every suggestion as a valuable opportunity to improve and enrich our work enormously.

 

Therefore, we have intended to be as punctual as we could to attend the decision of Revisions. Reviewer observations are highlighted using bold black font, our replies using a blue font and changes in existing sentences have been written in italic font.

 

Reviewer

 

1. The global applicative context discussed in the introduction appears somewhat unrealistic. lt can be envisioned that MFCs might be used to supply power to low-consumption electronic devices such as wireless sensors (temperature, humidity sensors, etc .), small medical instruments (e.g., glucose meters, pregnancy tests, etc.), future wearables, digital clocks, etc. One can assume that the amount of electricity generated by MFCs can (or could be adapted to) fit the power requirements of such devices. lt is rather doubtful, however, that MFCs will truly find applications in tablets, smartphones or laptops (especially a paper-based architecture such as the one presented here).

 

Dear reviewer, your observations have been taken into account now in the manuscript (Introduction section, line 18 to 28). Now, we mentioned the examples of the low-consumption electronic devices that you mentioned, and we coincided with your comment that it is rather doubtful that the MFCs will truly find applications in tablets, smartphones, or laptops.

 2.- In page 2, lines 56-60, the authors say:

" These enzymatic electrodes have the disadvantage of being very expensive, complex, in the immobilization method, and short duration; the above cause rapid decay of thefue/ ce// power output [20,21}. For this reason, in this work is proposed a paper-based mFC that use non-enzymatíc catalyst anode [...]" .

There is a relevant work (see [Rl]) that I cannot find in the references cited by the authors. In [Rl], the authors reported a paper-based fuel cell incorporating stacks of enzymatic electrodes and demonstrated the possibility to drive a small digital dock during 9 hours using an energy drink (Gatorade®). Measures of open circuit voltages far periods of time ranging from tens of hours to days were also reported.

Therefare, is there a real interest to use enzyme-less electrodes as argued by the authors? lndeed, it is likely that the small paper-strip they used will somehow be saturated by fluids and loase part of its capillary effect well befare the activity of enzymes would decay (hence I am not persuaded that the argument related to the limited lifetime of enzymatic electrodes really holds in this specific context).

In any case, the authors are invited to also compare their results to [Rl) and to provide a more detailed/quantified argumentation.

 

Dear Reviewer, we apologize for our out-of-context comment on enzyme materials. In some works (listed below) we have used enzymes as anodes electrodes in microfluidic fuel cells, which showed an excellent performance.

 a) Towards autonomous lateral flow assays: Paper-based microfluidic fuel cell inside an HIV-test using a blood simple as fuel. Dector, J. Galindo-de-la-Rosa, D. M. Amaya-Cruz, A. Ortíz-Verdín, M. Guerra-Balcázar, J. M. Olivares-Ramírez, L. G. Arriaga, J. Ledesma-García. International Journal of Hydrogen Energy. 2017. 42. 27979 – 27986.

b) Perspective use of direct human blood as an energy source in air-breathing hybrid microfluidic fuel cells. Dector, R. A. Escalona-Villalpando, D. Dector, V. Vallejo-Becerra, A. U. Chávez-Ramírez, L. G. Arriaga, J. Ledesma-García. Journal of Power Sources. 2015. 288. 70-75.

c) Glucose microfluidic fuel cell using air as oxidant. A. Escalona-Villalpando, A. Dector, D. Dector, a. Moreno-Zuria, S. M. Durón-Torres, M. Galván-Valencia, L. G. Arriaga. International Journal of Hydrogen Energy. 2016. 41. 23394 – 23400.

 

It is important to emphasize that we coincided with you that some materials (organic or inorganic) can show to be excellent catalysts for the oxidation reaction of glucose, fructose, maltose, and sucrose. However, each material (organic or inorganic) represents another option to use as a catalyst (this has now been placed in the text, Introduction section line 65 to 70) and a new contribution to materials science.

 

On the other hand, the reference [R1] mentioned by you has been added in our Introduction (Introduction section, line 58 to 63); in addition, the results presented in [R1] have been compared with ours in table 3 as you have suggested.

 3.- Similarly, there is a lack of arguments regarding why the authors chose specifically PtRu/C for their anode material. Since PtRu/C was deposited on a piece of carbon paper, other enzyme-less options could have been considered. For instance, highly porous gold electrodes have been reported to react to glucose, maltose, etc. (see for instance [R2]) . Since the paper focus on enzyme-less electrodes, a few other enzyme-less approaches should be cited and confronted to PtRu/C. lf the main criterion for selecting a PtRu/C ink turned out to be the cost, please be more specific: sorne quantification / cost estimation would be appreciated (at least for the PtRu/C).

 

Dear reviewer, we agree with you about the use of gold as a catalyst for the oxidation of saccharides. In previous works, our working group has also worked extensively with Au (synthesized and commercial), as well as bimetallic materials based on Au, to oxidize glucose present in human blood.

a) Non-conventional electrochemical techniques for assembly of electrodes on glassy carbon-like PPF materials and their use in a glucose microfluidic fuel-cell. Dector, N. Arjona, M. Guerra-Balcázar, J. P. Esquivel, F. J. Del Campo, N. Sabaté, J. Ledesma-García, L. G. Arriaga. Fuel Cells. 2014. 14.

 

b) Hybrid microfluidic fuel cell based on Laccase/C and Au Ag/C electrodes. López-González, A. Dector, F. M. Cuevas-Muñiz, N. Arjona, C. Cruz-Madrid, A. Arana-Cuenca, M. Guerra-Balcázar, L. G. Arriaga, J. Ledesma-García. Biosensors and Bioelectronics. 2014. 62. 221 – 226.

 

c) Fabrication and evaluation of passive SU8-based micro direct glucosa fuel cell. Dector, J. M. Olivares-Ramírez, V. M. Ovando-Medina, A. Sosa-Domínguez, A. L. Villa, A. Duarte-Moller, N. Sabaté, K. P. Esquivel, A. Dector. Microsystem Technologies. 2019. 25. 211-216.

d) Lateral flow assay HIV-based microfluidic blood fuel cell. M. Olivares-Ramírez, V. M. Ovando-Medina. A. Ortíz-Verdín, D. M. Amaya-Cruz, J. Coronel-Hernández, A. Marroquín, A. Dector. IOP Conference Series: Journal of physics. 2018. 1119. 1-6.

e) A new type of air-breathing photo-microfluidic fuel cell base don ZnO/Au using human blood as energy source. M. Ovando-Medina, A. Dector, I. D. Antonio-Carmona, A. Romero-Galarza, H. Martínez-Gutiérrez, J. M. Olivares-Ramírez. International Journal of Hydrogen Energy. 2019. 59. 31423 – 31433.

 

In fact, the main reason why we used PtRu/C was to explore new alternatives that in works reported by other authors would have shown promising results in the oxidation of saccharides [reference 25], but that it was necessary to further explore its application in real conditions such as the soft drinks employed as fuel in this work. Also, another reason for using PtRu/C is due to its present a minimal poisoning rate and a lower cost than Pt/C. (This has been indicated in the text, line 70).

 4.- Furthermore, the way the authors introduce their anode material is kind of misleading. In particular, in page 2, lines 58-59, the authors wrote:

" in this work is proposed a paper-based mFC that use non-enzymatic catalyst anode such as PtRu/C

[...]".

When one reads the paragraph, it seems that using PtRu/C as a catalyst is quite innovative. Nevertheless, PtRu/C was evaluated in a direct glucose fuel cell with glucose and fructose concentrations very similar to the ones discussed by the authors back to 2010 (see reference [301). This should be mentioned much more explicitly right from the introduction. Overall, the authors should discuss and compare much more their results with a paper like [30) (the analysis conducted should share similarities with results previously reported in [30) and if there are differences, they should be explained).

 

We did not want; when one reads the paragraph (on page 2, lines 67-68), it seems that using PtRu/C as an anode seemed misleading or innovative. Now we have rewritten the lines you mention to avoid such confusion (Line 58). Also, we have referenced in the introduction the works where PtRu/C has been used for the electro-oxidation of some saccharide in half-cell studies and in another type of fuel cell different from ours (such as the reference 25). All of the above, to subtract novelty from the PtRu, as you have suggested.

 5.- Overall I have sorne difficulties to be convinced by the logic and real interest of the configuration presented. In my view, using a paper strip makes a lot of sense when: i) the paper-MFC exploits ca­ laminar flows of reactants (i.e., the capillary action of the paper strip is used to get the 2 liquids flowing in parallel side by side like in a conventional microfluidic channel) ar; ii) only small amounts of reactants are available (e.g., the paper strip will "pump" a droplet of blood ar sweat towards the reactive areas).

Nonetheless, one majar advantages of the soft drinks is that they are available at a low cost in large quantitie s. So why bothering pipetting 15 µL of such a liquid on a paper-strip (especially considering the application targeted by the authors)? This appears somehow cont radicto ry. Would not it make more sense to use instead a specifically designed reservoir containing the electrodes in direct contact with the soft drink? Is it because the paper strip avoid troubles due to the gas bubbles present in drinks like Coca-Cola®? But if so, would not it be more interesting/convenient to just select other energy drinks/juices without any gas/bubbles?  Is the flow speed induced  by the capillary  action of the paper supposed to improve the overall performance? But to what extend? This should be investigated. 1 think such points should be discussed and/or quantified in a clearer and deeper manner so that the reader can grasp more easily the whole interest of the approach suggested by the authors.

 

Dear reviewer, in our previous works, we have experimentally determined the amount that is proportional to one drop of solution based on the information reported in the literature (one drop ranges from 0.3 µL to 50 µL). However, in our previous works, we have worked with blood, urine, glycerol or glucose solution in KOH. In the experimentation of these works, we have observed that the performance of the fuel cell does not change when adding between 15 to 50 µL. Therefore, our goal was to standardize the amount to use from our last two jobs:

 

• Dector, D.; Olivares-Ramírez, J.; Ovando-Medina, V.; Dominguez, A.S.; Villa, A.; Duarte-Moller, A.; Sabaté, N.; Esquivel, J.; Dector, A. Fabrication and evaluation of a passive SU8-based micro direct glucose fuel cell. Microsystem Technologies 2019, 25, 211–216.

 

• Evaluation of a Passive Anion‐Exchange Membrane Micro Fuel Cell Using Glycerol from Several Sources. M. Olivares‐Ramírez, A. Dector, J. A. Bañuelos‐Días, D. M. Amaya‐Cruz, A. Ortiz‐Verdín, O. Jiménez‐Sandoval, N. Sabaté, J. P. Esquivel. 2018. 19.

 

Without really thinking about what you mention about the availability and economy of soft drinks, which is very true and interesting. (This is now explained in the article to make it easier for readers to understand, section 2.4, line 130-135).

 

On the other hand, probably in future works, we can experiment with the configurations of a fuel cell and propose a reservoir containing the electrode in direct contact with the soft drink as you suggest.

 

Also, we agree with you that using non-carbonated beverages would be more convenient, such as works where some authors tested Gatorade, Vegetable juice, or Nutri express. However, we decided to try popular drinks that would not have been tested even with gas (even having the option to remove the gas) in order to contribute in the field of micro fuel cells.

 

Finally, regarding your last comment, we are currently preparing an article where we are studying the flow velocity induced by capillary action depending on the configuration of the fuel cell through simulation in ANSYS. The discussion suggested by you will be the purpose of this new work.

 6.- I am also a bit skeptical regarding the whole discussion about the temperature effect .

-First, when the authors say in conclusion (page 10, lines 220-221) :

"In addition, the use of co/d soft drinks showed a better performance than soft drinks at room temperature, which represents a more realistic approximation for a possible power supply user condition . "

I am not so convinced that it would be so convenient if it is needed to take a can right from the fridge to get better performance. In my view, if a fridge is somehow nearby, there are probably other easier ways to access powerful electricity sources around.

-Second, how long the 4ºC can be maintained with the paper-MFC proposed? Considering the small quantity of liquid {15 µL) deposited and the surface area exposed to ambient air, 1 guess the droplet of cold beverage should warm up pretty quickly. How long the best performance reported by the authors can be maintained?

Since the authors also highlight the advantages of stability over time of their PtRu/C electrodes compared to the enzymatic ones, they should include some measures (e.g., open circuit voltage) taken over long period of times and compare them to [R1] (far instance). ldeally, the measures in Figure 6 should be repeated several times (using the same paper-strip).

 

Dear reviewer, we were not clear about the application of a future device based on our work. We are now clear on the possible application in the text (line 130, section 2.4). When a user drinks a soft drink, it is more likely to be cold than hot. If for any reason, the user is in the street, a disco, some public transport, etc. the user could take his cold drink and recharge a low power device with a drop using the straw or a small stream of soda. This is an assumption; obviously, a lot of testing would be required to come up with an applicable device, we in this paper present only some basics and usage perspective assumptions. However, we have modified our conclusion to make it more objective and less subjective (line 242-244, section 4).

 

Dear reviewer, we consider that your doubt is adequate. We had not considered that observation. However, it is important to emphasize that the energy obtained in this type of fuel cell is immediate, but it is also important to take your point of view that a cold soda will heat up, and therefore performance will be compromised. We were experimentally always careful to maintain the mentioned temperatures, but in a real application, it would be difficult to do so. We have now clarified in our writing that the performance will be in relation to the temperature (line 242, section 4); in fact, it would be excellent to carry out in future works the effect of various temperatures on some fuels.

 

OCV time is directly related to the type of fuel cell used. For example, in a microfluidic or PEM fuel cell where the fluid is continuously injected, OCV or chronoamperometry experiments are performed for days. However, OCV experiments in this type of cells are related to the time in which the drop dries (10 to 15 minutes), and we stop receiving a response (this has now been clarified in the text, line 208).

Given the current contingency, it is impossible for us to agree to carry out experiments such as those requested. Our laboratories are closed.

On the other hand, the corresponding results to Figure 6 are expressed as mean values (n = 3) ± standard deviation and shown as a footnote in Table 2.

 

7.- The "assembly" should be enhanced. Right now, I am not sure that the paper-MFC can be used without the glass slide. And I am actually wondering if the glass side might affect somehow the temperature of the liquid absorbed by the paper-strip. ldeally, the MFC should be a stand-alone device.

 

Dear Reviewer, the glass slide adhesive was collocated with the objective of providing mechanical support for the paper-based µFC (as mentioned in the methodology). This was done from previous experiences where we have reported paper-based fuel cells without any support or with the plastic cassette of an HIV test used as support. We have observed that using or not using support does not influence the performance of the fuel cell; however, it is the first time that we make a report on temperatures with this type of fuel cells, so what you mention is correct. We should have been careful with this support that could transfer heat, in this sense, the thermal conductivity of glass is very low (1 W/m K) compared to copper (385 W/m K). However, in the laboratory, the temperatures of the soft drinks were carefully controlled, and there was no heat source nearby. In this sense, we believe that for a possible real application, heat-insulating materials should be used both in the support and in the integrated circuits.

 

 

Additional (more specific comments)

8.- Please consider that the paper-MFC reported may rise the interest of scientists outside the fields of electrochemistry or science materials (e.g., professionals in electrical engineering who could be interested to supply power to low-consumption electronic devices in an eco-friendly / sustainable manner). The authors are therefore kindly invited to define at least once the materials utilized in the paper, as all the acronyms (e.g., PtRu/C) may not be so trivial far everyone.

 

Dear Reviewer, Thank you for your observation. We have now modified the acronyms in the text.

Line 5 and 7

 9.- Please, consider repeating the dimensions of the electrodes directly in Figure 1 (a). Please provide a scale bar also in Figure 5b.

 

Dear reviewer, Figure 1a, and 5b have been modified.

 10.- In the caption of Figure 2: is it really 0.3 mM KOH? Or 0.3 M? 1 would actually combine Figure 2 and Figure 4 in a single figure with the 2 graphs side-by-side.

 

Dear reviewer. The caption Figure 2 has been modified. On the other hand, we appreciate your recommendation in the combination of Figure 2 and Figure 4; however, we believe it is crucial to have them separate for a better understanding of the reader.

 

11.- Please, consider adding some reference letters above the peaks discussed in Figure 2 and Figure 4 so that they can be identified in an easier manner when reading the text.

 

Dear reviewer, reference letters above the peaks discussed in Figure 2 and Figure 4 have been added.

 

12.-  Page 3 line 88-89: 1 cannot find any clear discussion regarding the composition of the different soft drinks in reference [24]. Are the authors sure they did not want to refer to [22] instead?

References mentioned in this reviewing report:

 

Thank you for your observation. The reference was changed in the lines 103

 

Author Response File: Author Response.pdf

Round 2

Reviewer 4 Report

Please see the report attached for details.

Comments for author File: Comments.pdf

Author Response

We are pleased to resend you the entitled article A Paper-based Microfluidic Fuel Cell Using Soft Drinks as a Renewable Energy Source for your consideration, with the good intention to be published in the section Special Issue "Advanced Materials and Technologies for Fuel Cells."

 

Therefore, we have intended to be as punctual as we could to attend the decision of Minor revisions. Reviewer observations are highlighted using bold black font, our replies using a blue font and changes in existing sentences have been written in italic font.

 

Dear Reviewer

 

The authors appreciate the corrections and hope that the answers enlisted below in this document fulfill the expectations. We considered every suggestion as a valuable opportunity to improve and enrich our work enormously.

 

Reviewer

 

1.- Introduction:

-The authors have agreed with the reviewer that targeting the power supply of laptops, smartphones and tablets with the paper-MFC was somewhat too optimistic. Nevertheless, they basically just replaced these keywords by the ones I mentioned (i.e.; wireless sensors, etc.). The problem is that the introductive context remains a bit awkward. Indeed the use of the “power bank” does moderately fit with the new low-consumption devices mentioned. Now, it appears rather unlikely that someone will try to use a paper-MFC to partially recharge a power bank that would be used to drive a sensor among a wireless network…And again, I doubt that generating a few hundreds of microwatts for 10-15 min would sufficiently recharge any kind of power bank… I can understand that an exact applicative contact may remain elusive to the authors at this stage of research. I believe, however, that the authors should spend more time to rewrite this important part. I think this would be beneficial for the paper. Otherwise, a too unrealistic/unlikely introductive context may somehow discredit the results presented

 

-Recently, a new enzymatic paper-based MFC using Pespi (also 7Up and other soft drinks) has been reported:

[R3] P. Rewatkar and S. Goel, “Microfluidic paper based membraneless biofuel cell to harvest energy from various beverages”, J. Electrochem. Sci. Eng. 10(1) (2020) 49-54;

 

I think the authors could also add such a paper and possibly compare their results (e.g., in Table 3) to show the interest of their electrode materials.

 

Dear reviewer, the reference [R3] mentioned by you has been added in our Introduction (Introduction section, line 63), in addition, the results presented in R3 have been compared with ours in table 3 as you have suggested.

 

List of changed:

 

Insert:

Line 3; “an emergency power source”

 

Delete:

Line 21;

The demand for low-consumption electronic devices such as wireless sensors, digital clocks, and small medical devices, to name a few, has stimulated an intensive mass production of these. In addition, an increase in the use of these devices causes high-energy consumption and, therefore, the need to change or frequently recharge the battery. The current problem is that their performance is limited by a power source, usually a rechargeable lithium-ion battery. Because power supply sockets are not always available, many solutions, as power banks, have been proposed to overcome these challenges. The power bank is a portable battery for mobile electronic devices that allows you to recharge these devices in emergencies. However, now the problem is the download for both the mobile device and the power bank.

 

Insert:

Line 20; “In this way,”

 

Delete

Line 23: “such as non-implantable medical devices”

 

Insert

Line 41: “Some relevant works evaluating paper–based µFCs under soft drinks are listed in Table 3 and described in the following lines:”

 

Insert:

Line 59: “Finally, in more recent research a miniature self-pumping paper-based enzymatic biofuel cell was constructed using glucose oxidase and laccase as anode and cathode, respectively, and soft drinks as fuels. The authors reported that power density obtained was attributed to the glucose contained in the soft drinks, which was fresh watermelon juice > 7up > Mountain Dew > Pepsi (14.5, 13.5, 12, and 6.15 µW cm-2) [R3] = [21].”

 

 

2.- Line 90-93:

Please, list all the materials used for experiments in subsection 2.1. For convenience, the Whatman paper, Nafion, etc. should be mentioned there as well.

 

Dear reviewer, all the materials have been added in section 2.1. Materials

 

Insert

Line 90: “Nafion 5%, “

 

Insert

Line 92: “Isopropyl alcohol and KOH acquired from J.T. Baker. Whatman filter paper, grade Fusion 5 was used for paper-based µFC construction.”

 

3.- Figure 1:

The authors have added a dimension in this figure, but it refers to the width of the paper strip. I was actually referring to the dimensions of the Toray electrodes so that there is no need to look for them in the text to know their surface. In my view, it also seems a bit confusing that the authors mention the “reaction zone” at the end of the paper strip. Should not it be where the electrodes are placed? Also in Figure 1, it seems that the anode and cathode completely overlap (i.e., they seem to be perfectly aligned one above the other with the paper strip in between). But in the picture of Figure 5b it does not seem to be so. Further details should be added.

 

Dear reviewer, Figure 1 has been modified taking into account your comments (Line 127).

 

4.- Line 226:

I cannot really figure out how were made the serial/parallel connections for the stacks. Did the authors fabricate 2/4 (separate) paper-MFCs which were connected between them using clips and wires? Or did the authors stack several Toray electrodes onto a single paper-MFC? If so how the electrodes were attached and connected between them? Further explanation should be provided.

 

Dear reviewer, thank you for your observation. We have now added:

 

Line 124: “On the other hand, for the construction of the 2-cell and 4-cell stacks, continuous pairs of electrodes were collocated (anode and cathode in the bottom and top, respectively) and rotated on the paper strip. Finally were connected following a serial connection using aluminum foil. Additionally, a tiny scheme has been inserting in Figure 8.”

5.- Conclusion:

I think the authors could elaborate just a bit further about their future research plans (including what they mentioned in their response to the reviewer like the ANSYS simulations, future tests conducted with larger quantity of liquids, etc.). This way, even though some of the points I referred to cannot be addressed in this particular paper, it would show that the authors are at least considering them for future research.

 

Dear reviewer, thank for you observation. We have now added the future ideas in the conclusion section,

 

Line 248: “commercial PtRu/C”

 

Line 251: “We foresee that combining the advantages in the materials field to use other catalysts on the paper-based µFCs would lead to better performances. The design of the presented paper-based µFC has been inspired in a single flow. In this sense, the study of the flow velocity induced by capillary action respect to the configuration of the fuel cell could simulate through software such as ANSYS.”

 

Line 258: “In addition, the use of other easily accessible fuels could be considered.”

Author Response File: Author Response.pdf