Carbon Dioxide Adsorption on Porous and Functionalized Activated Carbon Fibers

Round 1

Reviewer 1 Report

Manuscript Number: Applesci-485304-peer-review-v1

Title: Carbon dioxide adsorption on porous and functionalized activated carbon fibers.

Article Type: Research Paper

General comments:

Although authors focused on many characterization result discussion, but this manuscript needs significant improvement. Hence, I suggest major revision of this article at this stage. Moreover, my specific comments are below.

Specific comments:

1. ACF has already used for the removal of CO2. What is the new and novelty of this study?

2. In section 2. Where is the source of chemicals?

3. CO₂ adsorption capacity was 2.74 mmole/g at 25 ^oC. Is it significant removal capacity over the past studies? I found this is on average low removal capacity for removal of CO2. Although authors inserted one table, but they just listed some results, which showed the result close to their findings.

4. In section 3, I missed subtitles. Therefore, it seems to me jargon discussion.

5. ACF has itself higher surface area. After modification with TEPA why did surface area reduce? Should come under discussion.

6. What was the production temperature of ACF?

7. Figure 8 has been placed after conclusion. Is it right?

8. When comparing the published result, it is better to compare the similar conditions as well so that there remain a consistency.

Considering aforementioned concerns, I suggest this manuscript for major revision at this stage.

Comments for author File: Comments.pdf

Author Response

Manuscript ID: applsci-485304
Title: Carbon dioxide adsorption on porous and functionalized activated carbon fibers

Authors: Yu-Chun Chiang*, Cheng-Yu Yeh, Chih-and Hsien Weng

Thank the Reviewer for your valuable comments and suggestions on our submission. This manuscript has been carefully revised considering all the raised points. Our responses to the reviewer’s comments, point-by-point, and the changes made in the revised version are indicated below:

Specific comments of Reviewer 1:

1.      ACF has already used for the removal of CO2. What is the new and novelty of this study?

Responses:

Thanks for the reviewer’s suggestion. The studies regarding the CO₂ uptake of ACFs published in literature have almost always focused on the discussion of capacities and porosity. Only a few studies have talked about the effect of the chemical properties of ACFs on CO₂ adsorption, and almost no studies have investigated the role of surface functional groups on CO₂ adsorption performance. The related description has added in the last paragraph of the Introduction (lines 94-97) .

2.      In section 2. Where is the source of chemicals?

Responses:

Thanks for the reviewer’s suggestion. The sources of chemicals have been added in the revised manuscript, in lines 105, 106, and 112. .

3.      CO₂ adsorption capacity was 2.74 mmole/g at 25 ^oC. Is it significant removal capacity over the past studies? I found this is on average low removal capacity for removal of CO2. Although authors inserted one table, but they just listed some results, which showed the result close to their findings.

Responses:

Thanks for the reviewer’s consideration. The authors have tried to search the published papers related to CO₂ adsorption on ACFs. The information in Table 3 has been updated in the revised manuscript. The CO₂ adsorption capacities on KOH-treated ACFs were comparable to the most research or superior to the other porous carbons. (P. 10)

4.      In section 3, I missed subtitles. Therefore, it seems to me jargon discussion.

Responses:

Thanks for the reviewer’s suggestion. The subtitles have been introduced in the revised manuscript in section 3. There are 3.1. Textural and Chemical Characterization of the Adsorbents (line 171) and 3.2. CO₂ Adsorption Performance (line 264 ).

5.      ACF has itself higher surface area. After modification with TEPA why did surface area reduce? Should come under discussion.

Responses:

Thanks for the reviewer’s suggestion. The decrease in S_BET, V_t and V_mi of ACF after TEPA modification could be attributed to the fact that the micropores were totally or partially blinded by N-functionalities [11]. The TEPA has a bulk and long-chain structure. Thus, the graft of TEPA on ACFs could hinder the gas molecules from getting access to the pores or depositing on the carbon surface. Moreover, if the TEPA was attached inside the pores of the ACFs, the pore would be filled with TEPA molecules and the available adsorption surface or volume would be reduced. The discussion has been provided in lines 183-187.

6.      What was the production temperature of ACF?

Responses:

Thanks for the reviewer’s consideration. The ACF sample used as the raw material in this study was a commercial type of PAN-based ACF. According to the information provided by the manufacture, the activation temperature was ranged from 800 to 1000 °C.

7.      Figure 8 has been placed after conclusion. Is it right?

Responses:

Thanks for the reviewer’s consideration. The position of Figure 8 has been moved in front of the conclusion section, in P. 11.

8. When comparing the published result, it is better to compare the similar conditions as well so that there remain a consistency.

Responses:

Thanks for the reviewer’s suggestion. The information in Table 3 has been updated in the revised manuscript. (P. 10)

Author Response File: Author Response.pdf

Reviewer 2 Report

In the manuscript “Carbon dioxide adsorption on porous and functionalized activated carbon fibers”  the authors present their study on the use of activated carbon fibers in the adsorption of CO₂ and how this is affected by treating the fibers with KOH and by adding surface N-functional groups. The subject of the article would be of interest to the readership of Applied Sciences as it deal with the potential application of modified and functionalized carbon fibers in carbon dioxide capture and storage. While the results are interesting, I feel that this manuscript should undergo several changes before being published.

First of all, I would suggest that the authors should carefully proof-read their manuscript, as there are several shortcomings in the language of the manuscript. While these shortcomings, for the vast majority, are only a minor distraction, occasionally they can make the manuscript difficult to understand and the issue should be address before the manuscript is considered for publication.

The Results and Discussion section contains text that is either redundant, as it is already presented in the Introduction, or it would be better suited for the Introduction. For example: lines 250 to 258.

More specifically on the content of the manuscript:

- In the abstract, the authors seem to suggest that N-functional groups are the dominant factor affecting the efficiency of the carbon fibers in adsorbing carbon dioxide. However, in the Results and Discussion section, the authors clearly point out that the  most important parameter is not the presence of N-functional groups, but the presence of micro- and ultramicro-porosity. The abstract should be re-written to better reflect the conclusions of the study.

- I have a few concerns about the methodology for the characterisation of the porous structure of the materials under investigation using nitrogen adsorption isotherms:

* Why are the authors using BJH to investigate mesopores and NLDFT for the micropores? One of the advantages of using NLDFT or GCMC methods is that they can be used to model pores in both the micro- and meso-porous range. So the use of DFT should be sufficient to fully characterise the carbon fibers.

* It is also important to remember that NLDFT relies on the use of kernels of simulated gas adsorption/desorption isotherms onto the surface of simplified pores of different sizes. In order for this pore size distribution to be quantitatively meaningful it is important that the gas-solid interactions in the simulated system are as close as possible to those in the experimental system. What kernel did the authors use in this study and does this take into account the presence of nitrogen functional groups on the carbon surface? If the kernel does not take this into consideration, then the modelling results should be presented as a semi-quantitative representation of the pore-size distributions.

* There is a discrepancy between the total void volume and the sum of the meso-, micro and ultramicro-pores volume for all samples studied (table 1). How do the authors explain this?

* N₂adsorption isotherms are known to underestimate the presence of micro-pores, especially in materials such as activated carbon fibres. See Lozano-Castelló D, Cazorla-Amorós D, Linares-Solano A. Usefulness of CO2 adsorption at 273 K for the characterization of porous carbons. Carbon, 2004;42(7):1233–42.http://dx.doi.org/10.1016/j.carbon.2004.01.037

The authors should take this into account in their discussion and present their results as a semi-quantitative estimate of the void-space.

- The authors suggest that the use of TEPA leads to the blocking (or partial blocking)  of a large amount of micro-pore volume. I am do not quite understand what mechanism are the authors suggesting for this phenomenon and I would appreciate if the authors could expand on this.

- How is the deconvolution of the XPS peaks carried out?  I assume some standard software distributed with the instrument has been used, but this should be more clearly explained.

- Do the authors believe that a time of 45 s is sufficient to ensure full equilibrium of CO₂ adsorption to be reached? Has this been tested by comparing the results obtained for longer equilibration time?

- The vertical axis of figure 7b should show specific volume rather than surface area.

- The results presented in figure 8 seem to suggest that the decay in adsorption efficiency is mostly taking place after the first adsorption cycle. The authors should discuss this in more detail.

Author Response

Manuscript ID: applsci-485304
Title: Carbon dioxide adsorption on porous and functionalized activated carbon fibers

Authors: Yu-Chun Chiang*, Cheng-Yu Yeh, Chih-and Hsien Weng

Thank the Reviewer for your valuable comments and suggestions on our submission. This manuscript has been carefully revised considering all the raised points. Our responses to the reviewer’s comments, point-by-point, and the changes made in the revised version are indicated below:

Reviewer 2:

1.      First of all, I would suggest that the authors should carefully proof-read their manuscript, as there are several shortcomings in the language of the manuscript. While these shortcomings, for the vast majority, are only a minor distraction, occasionally they can make the manuscript difficult to understand and the issue should be address before the manuscript is considered for publication.

Responses:

Thanks for the reviewer’s consideration. The English used in this manuscript has been checked via English Editing Services from MDPI. (english-edited-9220, edited by Kayla Mackie)

2.      The Results and Discussion section contains text that is either redundant, as it is already presented in the Introduction, or it would be better suited for the Introduction. For example: lines 250 to 258.

Responses:

Thanks for the reviewer’s consideration. The content in lines 250 to 258 in the original manuscript has been moved to Introduction, lines 67-75 in the revised manuscript.

3.      In the abstract, the authors seem to suggest that N-functional groups are the dominant factor affecting the efficiency of the carbon fibers in adsorbing carbon dioxide. However, in the Results and Discussion section, the authors clearly point out that the  most important parameter is not the presence of N-functional groups, but the presence of micro- and ultramicro-porosity. The abstract should be re-written to better reflect the conclusions of the study.

Responses:

Thanks for the reviewer’s suggestion. The abstract has been modified. Please see the first page, lines 9-23.

4. I have a few concerns about the methodology for the characterisation of the porous structure of the materials under investigation using nitrogen adsorption isotherms:

4-1 Why are the authors using BJH to investigate mesopores and NLDFT for the micropores? One of the advantages of using NLDFT or GCMC methods is that they can be used to model pores in both the micro- and meso-porous range. So the use of DFT should be sufficient to fully characterise the carbon fibers.

Responses:

Thanks for the reviewer’s suggestion. The BJH method is the standard and commonly used procedure for calculating the pore size distribution in mesopore range. In the revised manuscript, the mesopore volumes and the pore size distribution with pores smaller than 50 nm were obtained by the DFT method. Please see lines 129-130, Table 1 (P. 5) and Figure 1 (P. 5).

4-2 It is also important to remember that NLDFT relies on the use of kernels of simulated gas adsorption/desorption isotherms onto the surface of simplified pores of different sizes. In order for this pore size distribution to be quantitatively meaningful it is important that the gas-solid interactions in the simulated system are as close as possible to those in the experimental system. What kernel did the authors use in this study and does this take into account the presence of nitrogen functional groups on the carbon surface? If the kernel does not take this into consideration, then the modelling results should be presented as a semi-quantitative representation of the pore-size distributions.

Responses:

Thanks for the reviewer’s consideration. The DFT model used in this study was provided by Micromeritics, where the slit pore model was used and assuming the pore walls of carbon had a uniform density. The kernel did not take into account the presence of functional groups on the carbon surface, thus the pore-size distributions were just a semi-quantitative representation.

4-3 There is a discrepancy between the total void volume and the sum of the meso-, micro and ultramicro-pores volume for all samples studied (table 1). How do the authors explain this?

Responses:

Thanks for the reviewer’s consideration. The micropores mean the pores smaller than 2 nm in diameter, and the ultramicropores represent the pores less than 0.7 nm in diameter. So the ultramicropore volume is a part of the micropore volume. Thus, the data in Table 1 is reasonable.

4-4 N₂adsorption isotherms are known to underestimate the presence of micro-pores, especially in materials such as activated carbon fibres. See Lozano-Castelló D, Cazorla-Amorós D, Linares-Solano A. Usefulness of CO2 adsorption at 273 K for the characterization of porous carbons. Carbon, 2004;42(7):1233–42.http://dx.doi.org/10.1016/j.carbon.2004.01.037

The authors should take this into account in their discussion and present their results as a semi-quantitative estimate of the void-space.

Responses:

l Thanks for the reviewer’s suggestion. In the study of Prof. Lozano-Castelló’ group, for the carbon molecular sieves with very narrow microporosity, carbon fibers (un-activated) and activated carbons with low degree of activation, they suggested that the combination of N₂ and CO₂ as adsorptives is important to understand and to improve the characteristics and performance of the carbons. This is a valuable suggestion to the authors for further study.

l In our study, the ACFs were highly activated. The first point of relative pressure of N₂ in the isotherm at 77K could achieve 5 ´ 10^-7. Moreover, the CO₂ uptakes were highly related with the micropore volume and the ultramicropore volume according to Figure 7b. Therefore, the microporosity obtained from N₂ adsorption isotherms at 77 K should be able to characterize the adsorbents and to predict the CO₂ adsorption performance for these adsorbents.

l In addition, several studies involving the discussion of ultramicropores were only got the information just from N₂ isotherms at 77 K. For instance: de Souza et al., Carbon (2013) 65, 334-340; Hu et al., Environmental Science & Technology (2011) 45, 7068-7074; Shen et al., Journal of Materials Chemistry (2011) 21, 14036-14040.

5. The authors suggest that the use of TEPA leads to the blocking (or partial blocking)  of a large amount of micro-pore volume. I am do not quite understand what mechanism are the authors suggesting for this phenomenon and I would appreciate if the authors could expand on this.

Responses:

Thanks for the reviewer’s suggestion. The TEPA has a bulk and long-chain structure. Thus, the graft of TEPA on ACFs could hinder the gas molecules from getting access to the pores or depositing on the carbon surface. Moreover, if the TEPA was attached inside the pores of the ACFs, the pore would be filled with TEPA molecules and the available adsorption surface or volume would be reduced. The discussion has been provided in lines 183-187.

6. How is the deconvolution of the XPS peaks carried out?  I assume some standard software distributed with the instrument has been used, but this should be more clearly explained.

Responses:

Thanks for the reviewer’s consideration. The XPSPEAK software (version 4.1, The Chinese University of Hong Kong, Hong Kong, China) was utilized for deconvolution of the XPS spectra, which was a nonlinear least squares curve-fitting program. Prior to quantification, a Shirley-type background was selected to be subtracted. After the baseline was subtracted, curve fitting was performed using a Gaussian function fitting program under an optimized peak shape. The above description has been complemented in the revised manuscript. (lines 140-142)

7. Do the authors believe that a time of 45 s is sufficient to ensure full equilibrium of CO₂ adsorption to be reached? Has this been tested by comparing the results obtained for longer equilibration time?

Responses:

Thanks for the reviewer’s concern. The authors have tried to conduct the experiments with an equilibrium time of 180 sec. The following figure shows the adsorption isotherms of CO₂ on nACF sample at 25 °C. It is believed that a time of 45 s is sufficient to ensure full equilibrium of CO₂ adsorption to be reached.

8. The vertical axis of figure 7b should show specific volume rather than surface area.

Responses:

Thanks for the reviewer’s pointing out the mistake. Figure 7b has been corrected, in P. 10 of the revised manuscript.

9. The results presented in figure 8 seem to suggest that the decay in adsorption efficiency is mostly taking place after the first adsorption cycle. The authors should discuss this in more detail.

Responses:

Thanks for the reviewer’s suggestion. The results presented in Figure 8 seem to suggest that the decay in adsorption efficiency was mostly taking place after the first adsorption cycle. It was expected that CO₂ molecules could get into all pores in the first adsorption cycle, especially the ultramicropores. When the adsorbents were desorbed in a vacuum, some CO₂ molecules could not escape effectively, most probably in the ultramicropores, because of the confinement effect. Thus, aACF, having the largest V_ultra, exhibited the highest decay of the adsorption capacity after the first adsorption cycle. The above description has been complemented in the revised manuscript. (lines 322-328)

Author Response File: Author Response.pdf

Reviewer 3 Report

The author did appreciable work by comparing three distinct Activated Carbon fibers for CO₂ adsorption. The Introduction is very clear with excellent literature review. In addition, the experimental procedure and results are unique and original. However, I have some points for the author consideration

1-      In Page 1 line 14, the sentence “KOH activation released nitrogen atoms on the surface of ACFs is not very clear for me. How KOH release nitrogen? explain, please

2-      Can you define ASAP, please?

3-      Page 4 line 165 you mentioned “The decrease in SBET, Vt and Vmi of ACF after TEPA modification could be attributed to the fact that the micropores were totally or partially blind by N-functionalities [10]” my question is: if you know from Lit that N-functionalities blocking  the pores why you used TEPA and it gave lower adsorption than ACF?

Author Response

Manuscript ID: applsci-485304
Title: Carbon dioxide adsorption on porous and functionalized activated carbon fibers

Authors: Yu-Chun Chiang*, Cheng-Yu Yeh, Chih-and Hsien Weng

Thank the Reviewer for your valuable comments and suggestions on our submission. This manuscript has been carefully revised considering all the raised points. Our responses to the reviewer’s comments, point-by-point, and the changes made in the revised version are indicated below:

Reviewer 3:

1-      In Page 1 line 14, the sentence “KOH activation released nitrogen atoms on the surface of ACFs is not very clear for me. How KOH release nitrogen? explain, please

Responses:

Thanks for the reviewer’s consideration. “KOH activation released nitrogen atoms on the surface of ACFs” has been modified as “KOH activation made nitrogen atoms easily able to escape from the surface of ACFs.”. (lines 14-15)

2-      Can you define ASAP, please?

Responses:

Thanks for the reviewer’s consideration. ASAP is the abbreviation of accelerated surface area and porometry. This has been supplemented in lines 123-124.

3-Page 4 line 165 you mentioned “The decrease in SBET, Vt and Vmi of ACF after TEPA modification could be attributed to the fact that the micropores were totally or partially blind by N-functionalities [10]” my question is: if you know from Lit that N-functionalities blocking  the pores why you used TEPA and it gave lower adsorption than ACF?

Responses:

Thanks for the reviewer’s concern. In the ref. [10] (it is ref. [11] in the revised manuscript), the authors used NH₃ and polyethylenimine (PEI) to modify the anthracite and they found the micropores were totally or partially blinded by N-functionalities. On the other hand, ref. [20] used TEPA to modify the carbon and they got a good CO₂ adsorption performance. But their carbons were reacted with SOCl₂, a toxic chemical, before TEPA modification and the products could be unstable under high temperature desorption. Thus we selected TEPA to directly modify the surface of ACF and followed a heat treatment. But the result was below expectation.

Author Response File: Author Response.pdf

Reviewer 4 Report

Dear authors,

After reading your manuscript I found some potential interest in the topic. 
However the research contains some flaws in most of its part and it must undergo a serious restyling before being considered pubblishable. I provide you some specific comments in order to improve it for your submission.

I sincerely wish you good luck for the manuscript pubblication.

Specific comments:
Introduction.

Line 36-39 should be rephrased and a better explanation of the phenomena described should be given.
Line 49 not clear the pore diameter ranges if the pore is 0.6 nm according to author distinction is both a small micropore and a unltra micropore should the small micropore have a lower bound ?

Line 56 What does PEI stand for ?

A scheme showing the proposed reaction mechanism should be added in order to show the functionalities imparted by the treatment.

In general the bibliography is quite updated, only 4 out of 41 of the cited sources are recent ( 2016 and on). Why is it so ? Did the author not took into consideration recent research ? Is the research topic not significant  for the scientific community.

The serious lack in the describing the state of the art makes difficult to understand the novelty of paper.

Materials and methods.

In general this section is not clear and does not provide in my opinion enough information to repeat the experiment. The authors do not clear state the number of replicates and the statistical analysis is missing.

Concerning characterization the author should have deeply described each one with a subsection in this way the reading is messy and difficult to follow.

The gas used in TGA analysis is not stated making difficult for the reviewer to interpret the results. For example in literature used TGA in air to obtain information on the surface composition  and oxygen content (10.1016/j.diamond.2017.10.013) here is not clear how to interpret the results since the methodology is not fully described.

Did the author performed FTIR analysis ? I well know that Infrared is a debated approach in the context of characterization of chemical modification of carbon compound, however some author succefully performed it. For example ( 10.3390/ma11081270, 10.3390/ma11101877 )
Furtherly it is a quite cheap and fast approach that would confirm or disprove the xps data and in my opinion must be included in the work or at least shown in complementary material. Also since the author state that some nitrogen is present as residual of PAN carbonization FTIR could be usefull for confirming and support the statement.

Results and discussion.

Table 1: No values of standard deviation are present, they should be included.

The Vme seems to have a different trend compared to other measured quantities. The author should comment on it.

Fig 2 SEM images are in general not very clear in general they may be substituted with bettere resolution ones. In the ACF one some spots are present which then disappear in KOH activated and reappear in TEPA modified ones. How is this explained.

Figure 3  the difference in nitrogen content are very small are they statistically significant ?

TGA: as stated before the author should state the gas in which it was performed and try to better relate TGA analysis to surface composition.

Table 3: This comparison is not quite fair since most of the sources taken into consideration are quite uptadated. The authors should include also more recent sources. Indeed the only recent source (23) displays better performances. Maybe the author should better comment it given also the growing interest on carbon based hierarchical materials in this field. 

Author Response

Manuscript ID: applsci-485304
Title: Carbon dioxide adsorption on porous and functionalized activated carbon fibers

Authors: Yu-Chun Chiang*, Cheng-Yu Yeh, Chih-and Hsien Weng

Thank the Reviewers for their valuable comments and suggestions on our submission. This manuscript has been carefully revised considering all the raised points. Our responses to the reviewers’ comments, point-by-point, and the changes made in the revised version are indicated below:

Reviewer 4:

Introduction.

1.      Line 36-39 should be rephrased and a better explanation of the phenomena described should be given.

Responses:

Thanks for the reviewer’s suggestion. The content in lines 36-39 “The porosity of activated carbons was highly associated with the activation temperature rather than the activation time [8]. It was found the adsorption capacity of gas at lower pressure was controlled by ultramicropores, but micropores and mesopores became dominant at higher pressure [8].” has been rephrased from the references. In the revised manuscript, this part has also been modified a little. In order to avoid the Introduction was too long, only the important conclusions were present.

2.      Line 49 not clear the pore diameter ranges if the pore is 0.6 nm according to author distinction is both a small micropore and a unltra micropore should the small micropore have a lower bound ?

Responses:

Thanks for the reviewer’s consideration. The description in Introduction was just abstracted and rephrased from literature. Sometimes, the denotation or definition was not consistent. In this study, micropores mean the pores smaller than 2 nm in diameter, and the ultramicropores represent the pores less than 0.7 nm in diameter. The ultramicropore volume is a part of the micropore volume. In general, there is no lower bound in this definition.

3.      Line 56 What does PEI stand for ?

Responses:

Thanks for the reviewer’s concern. PEI is the abbreviation of polyetherimide. This has been supplemented in line 56.

4.      A scheme showing the proposed reaction mechanism should be added in order to show the functionalities imparted by the treatment.

Responses:

Thanks for the reviewer’s suggestion. This paper is not the first one to functionalize the surface of the carbons using TEPA. Some schematic diagrams have been proposed in literature. Since the CO₂ adsorption performance on TEPA-modified ACFs was below expectation in this study, it is not a good timing to proposed the reaction mechanism.

5.      In general the bibliography is quite updated, only 4 out of 41 of the cited sources are recent ( 2016 and on). Why is it so ? Did the author not took into consideration recent research ? Is the research topic not significant  for the scientific community.

Responses:

Thanks for the reviewer’s concern. This paper was based on ACFs, thus the references were focused on this porous material. The increase in the atmospheric CO₂ level has been almost guaranteed as being responsible for global warming and climate change. Thus, CO₂ capture is very important issue. Recently, several special structures of carbon materials have been studied to serve as CO₂ adsorbents. However, some have complicated manufacturing processes, some were expensive, and there was often a challenge in realizing mass production and commercialization. In terms of practical applications, ACFs should be a feasible adsorbent, and how to improve their CO₂ uptake is a relevant question.

6.      The serious lack in the describing the state of the art makes difficult to understand the novelty of paper.

Responses:

Thanks for the reviewer’s concern. The studies regarding the CO₂ uptake of ACFs published in literature have almost always focused on the discussion of capacities and porosity. Only a few studies have talked about the effect of the chemical properties of ACFs on CO₂ adsorption, and almost no studies have investigated the role of surface functional groups on CO₂ adsorption performance. The related description has added in the last paragraph of the Introduction (lines 94-97) .

Materials and methods.

7.      In general this section is not clear and does not provide in my opinion enough information to repeat the experiment. The authors do not clear state the number of replicates and the statistical analysis is missing.

Responses:

Thanks for the reviewer’s suggestion. The content of Materials and Methods in the revised manuscript has been updated. Figure 8 shows the ten successive cyclic adsorption/desorption tests. This experiment lasted for about three months. In this paper, several experiments were rather time-consuming and expensive. Thus statistical analysis was not applied in this study.

8.      Concerning characterization the author should have deeply described each one with a subsection in this way the reading is messy and difficult to follow.

Responses:

Thanks for the reviewer’s suggestion. The content of Materials and Methods in the revised manuscript has been updated. Because these techniques are familiar and commonly used methods for characterization of the porous carbonaceous materials. In general, most papers describe this part in one paragraph.

9.      The gas used in TGA analysis is not stated making difficult for the reviewer to interpret the results. For example in literature used TGA in air to obtain information on the surface composition  and oxygen content (10.1016/j.diamond.2017.10.013) here is not clear how to interpret the results since the methodology is not fully described.

Responses:

Thanks for the reviewer’s suggestion. The atmosphere was air in TGA analysis. The related description has added in the Section 2.2 (P. 3, line 144).

10.  Did the author performed FTIR analysis ? I well know that Infrared is a debated approach in the context of characterization of chemical modification of carbon compound, however some author succefully performed it. For example ( 10.3390/ma11081270, 10.3390/ma11101877 )
Furtherly it is a quite cheap and fast approach that would confirm or disprove the xps data and in my opinion must be included in the work or at least shown in complementary material. Also since the author state that some nitrogen is present as residual of PAN carbonization FTIR could be usefull for confirming and support the statement.

Responses:

Thanks for the reviewer’s suggestion. Actually, the authors have tried to conduct FTIR analysis for ACFs, but the results were fail. In addition, we have no authorities to operate this machine. So, these is a problem to get good experimental results.

Results and discussion.

11.  Table 1: No values of standard deviation are present, they should be included.

Responses:

Thanks for the reviewer’s suggestion. The standard deviations were added in Table 1 for the specific surface areas. (P. 5)

12.  The Vme seems to have a different trend compared to other measured quantities. The author should comment on it.

Responses:

Thanks for the reviewer’s suggestion. Vme of nACF was a little higher than that of ACF, which could be attributed to the blocking of macropores by the N-groups and forming the mesopores.

13.  Fig 2 SEM images are in general not very clear in general they may be substituted with bettere resolution ones. In the ACF one some spots are present which then disappear in KOH activated and reappear in TEPA modified ones. How is this explained.

Responses:

Thanks for the reviewer’s suggestion. These images were taken in a higher magnification to examine the porosity on the surface. Some spots deposited on the surface of as-received fibers, and these disappeared after KOH activation. This could be attributed to the removal during the pore generation processes. It is not appropriate to say that the spots reappear on the TEPA-modified ACFs because aACF and nACF were independent. The spots on nACF might be due to the graft of TEPA.

14.  Figure 3  the difference in nitrogen content are very small are they statistically significant ?

Responses:

Thanks for the reviewer’s concern. From the point of view of microanalytical method, we believed the data were significant, which depends on the accuracy and precision of the instrument.

15.  TGA: as stated before the author should state the gas in which it was performed and try to better relate TGA analysis to surface composition.

Responses:

Thanks for the reviewer’s suggestion. The atmosphere was air in TGA analysis. The related description has added in the Section 2.2 (P. 3, line 144).

16.  Table 3: This comparison is not quite fair since most of the sources taken into consideration are quite uptadated. The authors should include also more recent sources. Indeed the only recent source (23) displays better performances. Maybe the author should better comment it given also the growing interest on carbon based hierarchical materials in this field. 

Responses:

Thanks for the reviewer’s suggestion. The information in Table 3 has been updated in the revised manuscript. In this paper, the carbon materials were focused on ACFs.

Author Response File: Author Response.pdf

Reviewer 5 Report

Dear Chun Chiang, Cheng-Yu Yeh and Chih-Hsien Weng

    It was a pleasure for me to read Your paper, because it presents a very good level of scientific, and it was very well written in terms of editional side.

    I would like to recommend it for publication in Applied Sciences Journal.

Author Response

Thanks for the reviewer’s comments.

Round 2

Reviewer 1 Report

Accept

Reviewer 2 Report

The authors have responded to most of the issues raised in my review, so I would like to recommend the manuscript for publication in Applied Sciences.

Reviewer 4 Report