Novel biomass-derived smoke-like carbon as a supercapacitor electrode material

In this present work, smoke-like carbon was successfully fabricated from a bio-waste fungal substrate crude polysaccharide for the first time. The as-prepared products possess smoke-like structures, ultra-high specific surface area (SBET: 2160 m2 g−1) and a high content of micropores (microporous surface area of 60%, with a nanopore size of 0.70 nm), which can increase the specific capacitance, representing a wonderful structure for electrochemical energy storage devices. The as-prepared sample displayed an excellent specific capacitance of 152 F g−1 at 5 A g−1 in the three-electrode configuration and exhibited maximal densities of 6.8–10.2 W h kg−1 under power outputs of 253.4–24.3 kW kg−1. We believe that this work demonstrates a simple, green and low-cost route by using agricultural residue to prepare applicable carbon materials for use in energy storage devices.

The manuscript is basically interesting. The problems should be clear before published. 1. BET calculation requires more details. Such as which device was used? 2. Miss-spell pg5 line 47 "ant" should be "and", also same page line 26 "respecticely" 3. Power density supposed to be calculated by discharge time, not time difference. 4. Please add "and" between 0.927 -1.162 cm3/g; pg6 line 27. 5. Please add a,b,c,d to Figure 6. 6. I would like to see 1000 charge-discharge cycle performance for single electrode and symmetric device. 7. SEM pictures after the 1000 cycles would be beneficial. 8. Authors did not mention about the electrode preparation methods. What subtract material used, what was the mass loading of the electrodes? 9. Please try also asymmetric device, it would be interesting to see one side active carbon, carbon black or composites and the other side smoke-like carbon. Also provide 1000 cycles for the device as well. The editor assigned to your manuscript has now received comments from reviewers. We would like you to revise your paper in accordance with the referee and Subject Editor suggestions which can be found below (not including confidential reports to the Editor). Please note this decision does not guarantee eventual acceptance.
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When submitting your revised manuscript, you must respond to the comments made by the referees and upload a file "Response to Referees" in "Section 6 -File Upload". Please use this to document how you have responded to the comments, and the adjustments you have made. In order to expedite the processing of the revised manuscript, please be as specific as possible in your response. ********************************************** RSC Associate Editor: Comments to the Author: (There are no comments.) RSC Subject Editor: Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) The authors synthesised the smoke-like carbon from a bio-waste fungal substrate crude polysaccharide and used as the electrode material of supercapacitors. The new finding of this work is in the synthesis part of this material. I recommend Accept after revision; Comments 1. It is well known that biomass consists of impurities. The elemental analysis of raw materials and as-prepared materials e.g., WDXRF is needed.
2. For the electrochemical result, Coulombic and energy efficiencies are needed.
3. Self-discharge testing is also needed. Comments to the Author(s) Authors derived the smoke-like carbon for supercapacitors applications. The electrodes provide the specific capacitance as high as 152 F/g. Also the symmetric device showed good performance. The manuscript is basically interesting. The problems should be clear before published. 1. BET calculation requires more details. Such as which device was used? 2. Miss-spell pg5 line 47 "ant" should be "and", also same page line 26 "respecticely" 3. Power density supposed to be calculated by discharge time, not time difference. 4. Please add "and" between 0.927 -1.162 cm3/g; pg6 line 27. 5. Please add a,b,c,d to Figure 6. 6. I would like to see 1000 charge-discharge cycle performance for single electrode and symmetric device. 7. SEM pictures after the 1000 cycles would be beneficial. 8. Authors did not mention about the electrode preparation methods. What subtract material used, what was the mass loading of the electrodes? 9. Please try also asymmetric device, it would be interesting to see one side active carbon, carbon black or composites and the other side smoke-like carbon. Also provide 1000 cycles for the device as well. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. The main corrections in the paper and the responds to the reviewer's comments are as following:

Coments:
Reviewer1: 1， It is well known that biomass consists of impurities. The elemental analysis of raw materials and as-prepared materials.

Review2:
1. BET calculation requires more details. Such as which device was used? 2. Miss-spell pg5 line 47 "ant" should be "and", also same page line 26 "respecticely" 3. Power density supposed to be calculated by discharge time, not time difference. 4. Please add "and" between 0.927 -1.162 cm3/g; pg6 line 27. Please add a,b,c,d to Figure 6.

5.
I would like to see 1000 charge-discharge cycle performance for single electrode and symmetric device.
6. SEM pictures after the 1000 cycles would be beneficial. 7. Authors did not mention about the electrode preparation methods. What subtract material used, what was the mass loading of the electrodes?
8. Please try also asymmetric device, it would be interesting to see one side active carbon, carbon black or composites and the other side smoke-like carbon. Also provide 1000 cycles for the device as well.
Appendix A Response to reviewer1: 1. It is well known that biomass consists of impurities. The elemental analysis of raw materials and as-prepared materials.
We are very sorry for our unclear report in the basic characterization of raw material and as-prepared materials. For the electrode material of supercapacitor, the influence of its elemental composition on its EDLC behavior is enormous. The elemental analysis is needed. As shown in Fig.1a, the crude polysaccharide has a natural three-dimensional As shown in Fig S2(b), after 5000 cycles, the CPC600 has a certain agglomeration phenomenon, and its flaky structure was not obviously destroyed after thousands of cycles. Therefore, the specific capacitance of CPC600 still retains 92.1.% after 5000 cycles, as shown in fig s2(a). As for the coulombic efficiency, after 5000 cycles, the coulomb efficiency is generally stable at 90%, which indicates that CPC has good cycle stability.
3. Self-discharge testing is also needed. (1) Where V1 and V2 are the voltages at time t1 and t2 respectively, C is the capacitance of supercapacitor in Farad. The leakage current of the supercapacitors was also calculated using (2) (2) Where IL is the leakage current, dV/dt is the slope of the curve and C is the  Fig S2 (c) shows the relation between the log (self-discharge voltage) as a function of time. Generally, discharge through an ohmic leakage leads to a declining linear relation between log V and t. From Fig S2(c), it is evident that, even after neglecting the initial drop due to ESR, the curve does not follow a perfect linear path.
Hence the selfdischarge cannot be only due to the ohmic leakage pathways between the two electrodes. There can be some additional mechanism which also contributes to the self-discharge. In order to understand whether there is any diffusion-controlled mechanism, we have also plotted V vs t1/2. Fig S3(d) shows the relation between selfdischarge voltage decline V and t1/2. In general, due to diffusion controlled faradaic leakage current in carbon based supercapacitors, the open circuit voltage shows a linear declining relation with t1/2. As obvious from Fig S3(d), the V and t1/2 have a better linear relation particularly in the slow discharge region. This clearly indicates that the diffusion-controlled mechanism will be the predominant mechanism in self discharge of these supercapacitors in addition to the ohmic leakage [1]. From fig S3(c), it is clear that V vs logt curves shows no linearity. Thus it can be confirmed that there is no contribution from overcharging in the self-discharge of graphene supercapacitors.
Hence it is clear that the self-discharge in these graphene supercapacitors is controlled by the combined contribution from potential controlled model due to ohmic leakage and diffusion-controlled model due to charge re-distribution phenomenon.

Any effects or charge contribution from the surface impurity or functional group?
As for FT-IR for the somke-like carbon, the broad peak at 3500 cm -1 is the intermolecular and intramolecular -OH group stretching vibration peak of the fungus polysaccharide, and the double peak at 2900 cm-1 is the CH2 group in the fiber, and the peak at 1650 cm -1 indicates C=O. The symmetric stretching vibration, the peak at 1100 cm -1 is the stretching vibration of the C-O-C group. The peak at 1000 cm -1 indicates the presence of a pyranose ring in the polysaccharide of the fungus. After carbonization and pickling, potassium ions almost completely evaporate, so the existence of potassium is one of the key points of our further study [2].
We are very glade to quote these two works in this manuscript. Sethuraman CPCs exhibits very high N2 uptake in the low-pressure region (p/p0 < 0.01), suggesting the very large amounts of microporosity. The calculated BET specific surface area, total pore volume, and mean pore diameter of CPC are 2377 m2 g-1, 1.50 cm3 g-1, and 2.53 nm, respectively. For the pore size distribution in the micropore region (calculated from NLDFT method) as shown in Fig S4, the CPC displays a single pore size of 1.93 nm, whilst in the mesopore region (calculated from BJH method), the average pore size is 0.7 nm as shown in the inset image.
We are very sorry for the confusion for editors and reviewers caused by our negligence in the writing process. We have already checked the manuscript and corrected the above mistakes.

3.
Power density supposed to be calculated by discharge time, not time difference.
Thanks to the reviewer for pointing out, we are sorry for the unclear report in the manuscript. The △t means the discharge time instead of difference, we have changed △ t (s) to t (s) in our manuscript to make it more clear.

4.
Please add "and" between 0.927 -1.162 cm3/g; pg6 line 27.Please add a,b,c,d to Figure 6. We are very sorry for the confusion for editors and reviewers caused by our negligence in the writing process. We have already checked the manuscript and corrected the above mistakes.

5.
I would like to see 1000 charge-discharge cycle performance for single electrode and symmetric device. As shown in Fig 3b, after 5000 cycles, the CPC600 has a certain agglomeration phenomenon, and its flaky structure was not obviously destroyed after thousands of cycles. Therefore, the specific capacitance of CPC600 still retains 92.1.% after 5000 cycles, as shown in fig 3(a). As for the coulombic efficiency, after 5000 cycles, the coulomb efficiency is generally stable at 90%, which indicates that CPCs has good cycle stability [4] .
6. SEM pictures after the 1000 cycles would be beneficial.
As shown in Fig 3, after 5000 cycles, the CPC600 has a certain agglomeration phenomenon, and its flaky structure was not obviously destroyed after thousands of cycles. Therefore, the specific capacitance of CPC600 still retains 92.1.% after 5000 cycles, as shown in fig s2(a). As for the coulombic efficiency, after 5000 cycles, the coulomb efficiency is generally stable at 90%, which indicates that CPC has good cycle stability.

7.
Authors did not mention about the electrode preparation methods. What subtract material used, what was the mass loading of the electrodes?
We are very sorry for our unclear report on the preparation methods of the electrodes and the solid-state symmetric supercaoacitors, now add as follows:

1, Fabrication of electrodes and solid-state symmetric supercapacitors
Nickel foam was first cut into rectangle sheets (20 mm * 10 mm) and treated with acetone, diluted HCl and deionized water each for 10 min ultra-sonication. A mixture containing 80 wt% active material, 10 wt% conductive carbon black and 10 wt% polyvinylidene fluoride (PVDF) was well grinded with appropriate amount of N-methyl- as-prepared electrolyte for 5 min before assembly. Then, the electrodes were picked out and transferred to a fume hood at room temperature for 1 h to vaporize the excess water. Finally the electrodes were pressed together under the pressure of 1 MPa for 10 min and sealed with plastic wrap to assemble the solid-state supercapacitor. The total mass of active materials for a symmetric supercapacitor was 6.0 mg [3] .

2, Fabrication of coin-type symmetric supercapacitors in ionic liquid electrolyte
The electrochemical performances of the PGBC-based symmetric supercapacitors in ionic liquid electrolyte were measured in a two-electrode cell configuration (CR2032-type coin cell). The electrodes were prepared by coating the aforementioned mixture containing active materials onto current collectors (nickel foam) with loading mass of about 8 mg/cm2, then dried in vacuum at 120 ºC for 8 h and pressed at 10 MPa. A neat ionic liquid of 1-Ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM TFSI) was used as the electrolyte, and a polypropylene membrane (MPF30AC, NKK, Japan) as the separator. The coin-type supercapacitors were finally assembled in an argon-filled glove box.

8.
Please try also asymmetric device, it would be interesting to see one side active carbon, carbon black or composites and the other side smoke-like carbon. Also provide 1000 cycles for the device as well.
Thanks to the reviewers for making such interesting suggestions and making our research more comprehensive. We have tried to assemble the activated carbon and acetylene respectively with CPC600s into devices and studied their electrochemical properties. We are sorry that due to the mistakes in the experimental operation, we can't finish the data before June 2. We will study the cyclic stability of asymmetric devices in future research and looking forward to further communication with you.