Dopamine-derived nitrogen-doped carboxyl multiwalled carbon nanotube-modified graphite felt with improved electrochemical activity for vanadium redox flow batteries

Improving the electrochemical activity of electrodes is essential to the development of vanadium redox flow battery (VRFB). In this work, we prepared a novel electrode with the modification of nitrogen-doped carboxyl multiwalled carbon nanotubes using dopamine as an eco-friendly nitrogen source (carboxyl MWCNT@PDAt). Characterization and electrochemical measurements reveal that the synthesized carboxyl MWCNT@PDAt-modified graphite felt electrode (carboxyl MWCNT@PDAt/GF) exhibits excellent electrochemical performance toward VO2+/VO2+ reaction. Superior battery performance was obtained with the energy efficiency of 80.54% at a current density of 80 mA cm−2. Excellent durability of the carboxyl MWCNT@PDAt/GF electrode was confirmed by long-term charge/discharge tests. The enhanced reaction kinetics of VO2+/VO2+ is ascribed to the synergetic effect of oxygen and nitrogen containing groups on graphite felt surface and the presence of nitrogen-doped carboxyl multiwalled carbon nanotubes (MWCNT). The facile approach proposed in this paper provides a new route to the fabrication of electrode with excellent performance for VRFB.


15-Apr-2020
Dear Dr Li: Title: Dopamine-derived nitrogen doped carboxyl MWCNT modified graphite felt with improved electrochemical activity for vanadium redox flow batteries Manuscript ID: RSOS-200402 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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1. The authors should provide cycle result more than 100 cycles. 2. The authors should provide rate capability result.

Reviewer: 2
Comments to the Author(s) This paper presented the nitrogen and sulfur co-doped reduced graphene oxide as catalyst for VRFB application. Thiourea, which is economical, was used as nitrogen and sulfur sources. The manuscript is well organized and the investigation is innovative. However, there are still several problems needed to be solved. 1. The used heat treatment temperature for the composite electrode is 500, 700 and 900 ℃, respectively. Why not treat the NS-rGO/GF at a higher temperature? A higher treatment temperature may lead to a higher degree of carbonization, resulting in an improvement in conductivity. 2. What is the effect of sulfur element on the improved performance of composite electrode? 3. Why the co-doping of nitrogen and sulfur elements has a synergistic effect on the improvement in catalytic activity, resulting in the enhancement in cell performance? Please give more detail explanation. 4. There are several mistakes in the paragraph. For instance, "0.01 to 100 kHz" should be modified to "0.01 to 1╳105 Hz". Additionally, "oxidized sulfur groups (-Son-)" should be revised to "oxidized sulfur groups (-SOn-)" 5. Do you think rGO-NS can be well dispersed in DMF at such a high concentration (100 mg rGO-NS in 50 mg DMF) ? Ask the author to give some reasonable feedback.

Reply:
The rate capability for cell with carboxyl MWCNT@PDA-900 has been tested at current densities from 80 to 160 mA/cm 2 , as shown in Fig. 2. The details are shown as follows: The rate performance of cell with carboxyl MWCNT@PDAt-900/GF at current densities from 80 to 160 mA/cm 2 is shown in Fig. 2. It is obvious that the discharge capacity decreases with current density increasing. The reason is ascribed to the higher polarization at a higher current density. The cell with carboxyl MWCNT@PDAt-900/GF exhibits an increased discharge capacity than that with pristine GF at each current density. Especially, the capacity of cell with carboxyl MWCNT@PDAt-900/GF at the current density of 160 mA/cm 2 is 545.4 mAh, which is 163.4 mAh higher than that with pristine GF. Therefore, VRFB based on carboxyl MWCNT@PDAt-900/GF shows enhanced rate capacity than that based on pristine GF, which may be owing to the significantly reduced mass transfer and charge transfer resistances resulting from the introduced oxygen and nitrogen containing functional groups. Figure. 2 Rate performance at current densities from 80 to 160 mA/cm 2 .
Reviewer: 2 Q1. The used heat treatment temperature for the composite electrode is 500, 700 and 900 ℃, respectively. Why not treat the NS-rGO/GF at a higher temperature? A higher treatment temperature may lead to a higher degree of carbonization, resulting in an improvement in conductivity.

Reply:
Thank you for your recommendation. In this paper, the used heat treatment temperature were 500, 700 and 900 ℃. Even though a higher treatment temperature may lead to a higher degree of carbonization of composite electrode, it also consumes more energy.
Furthermore, the electrode performance is not only related to electrode conductivity, but also depended on the catalytic activity of electrode. The functional groups on electrode surface determine the catalytic activity of electrode. During heat treatment process, the decomposition of functional groups may lead to the decrease in active sites.
Hence, the electrode performance is jointly controlled by the inherent conductivity and surface functional groups of electrode. Therefore, comprehensively considering of consumed energy, conductivity and amount of functional groups, the highest temperature used in this manuscript was 900 ℃.

Q2.
What is the effect of sulfur element on the improved performance of composite electrode?

Reply:
Thank you for your review. For the prepared carboxyl MWCNT@PDAt modified graphite felt electrode, the main doped element was nitrogen. According to the reported literature and our characterization results, the doped nitrogen element has positive effect on the performance of composite electrode. In detail, the state of nitrogen element in the composite electrode can be divided into pyridinic-N, pyrrolic-N and and graphitic-N. Resulting from the analysis of N1s core-level spectra, the relatively atomic fraction of the graphitic-N in carboxyl MWCNT@PDAt-900 is 29.01%, which is higher than that of carboxyl MWCNT@PDA. The improved content of graphitic-N is benefit for the enhancement in electrochemical performance [1,2]. Compared with carboxyl MWCNT, the carboxyl MWCNT@PDAt-900 shows a significant increase in the content of nitrogen. In charge-discharge test, the cell equipped with carboxyl MWCNT@PDAt-900/GF performs a higher energy efficiency than that equipped with carboxyl MWCNT -900/GF. Therefore, the improved energy efficiency for cell with carboxyl MWCNT@PDAt-900/GF is mainly ascribed to the doped nitrogen element, resulting in an improvement in kinetics of vanadium redox reaction and mass transfer rate of vanadium ions. Q3. Why the co-doping of nitrogen and sulfur elements has a synergistic effect on the improvement in catalytic activity, resulting in the enhancement in cell performance?
Please give more detail explanation.

Reply:
In this paper, the primary elements in the synthesized composite electrode is nitrogen and oxygen. Owing to the introduction of oxygen functional groups and nitrogendeficient skeletons, carboxyl MWCNT@PDAt-900/GF shows a significant improvement in electrochemical performance. The appearance of N atoms not only forms vacancies and defects in the composite electrode, but also changes the charge distribution between nitrogen and carbon atoms, resulting in the improvement in mass transfer rate and electron transfer kinetics of vanadium redox couples [1][2][3].
Furthermore, the oxygen functional group can provide more active sites for vanadium redox reaction, especially the content of C=O is directly related to the number of active sites [4]. During charging process, the positively charged vanadium ions (e.g. VO 2+ ) are easily absorbed by the negatively charged nitrogen atoms to form N-V bond. After the oxidation or reduction process, the reaction product (e.g. VO 2 + ) is easily diffused from the active site into the electrolyte solution [5]. Therefore, the introduced oxygen functional groups and nitrogen defects in the nitrogen-doped carboxyl MWCNT@PDAt-900/GF have a synergistic effect on the enhancement in VRFB performance.

Reply:
Thank you for your suggestion. The spelling mistakes have been modified in the manuscript and marked in red font. For instance, "0.01 Hz to 100 kHz" has been modified to "0.01 to 1╳10 5 Hz". The sentence "Q represent the electric double layer capacitance between electrode and electrolyte interface, and the Warburg impedance W concerned with vanadium ions diffusion" has been modified to "Q represents the electric double layer capacitance between electrode and electrolyte interface, and the Warburg impedance W is concerned with vanadium ions diffusion".
Q5. Do you think rGO-NS can be well dispersed in DMF at such a high concentration (100 mg rGO-NS in 50 mg DMF)? Ask the author to give some reasonable feedback.

Reply:
Thank you for your review. The prepared catalysts can be well dispersed in DMF at high concentration after ultrasonic treatment for 1h followed by magnetic stirring for 2h, as shown in Fig. 3. No obvious deposition was found after remained at room temperature for 12 h. Therefore, it can be concluded that the catalysts have been uniformly and stably dispersed in DMF.