Comment www.acsami.org
Comment on the Comment on “Ultrahigh Performance Supercapacitor from Lacey Reduced Graphene Oxide Nanoribbons” Vikrant Sahu, Shashank Shekhar, Raj Kishore Sharma,* and Gurmeet Singh* Department of Chemistry, University of Delhi, Delhi 110007, India u et al. in their comment on the article “Ultra high performance supercapacitor from lacey reduced graphene oxide nanoribbons” showed disagreement on the capacitance values.1 The comment by X. Wu et al. ignores the details given in our article and the conclusions thus drawn are not warranted as discussed below. Our article demonstrates chemical unzipping of the MWCNTs with hole formation in an individual nanoribbon. These holes were further exaggerated during high-temperature activation and reduction, leading to significantly enhanced edge density and accordingly the material named as lacey reduced graphene oxide nanoribbons (LRGONR). Wu et al. need to accept that LRGONR is a modified graphene (edge enriched). Besides HRTEM, an intense D peak in the Raman spectra (Figure 2b in the article) confirms high edge density.2 Wu et al. used the intrinsic areal capacitance (21 μFcm−2) of carbon as the upper limit to calculate specific capacitance of LRGONR.3 They ignored the fact that 21 μFcm−2 was obtained purely from the basal plane by masking graphene edges. This limit cannot be applied on LRGONR because there is no consideration of edge contribution. The literature suggests that edges and surface roughness contribute ∼300% of the quantum capacitance of carbon.4 Graphene edges are found to contribute 4 orders of magnitude (1.0 × 105 μFcm−2) higher capacitance than those of basal planes (4.0 μFcm−2).5 Edges contribute enormously high capacitance due to convergent diffusion effects.5 High capacitance at edges is attributed largely to accumulation of ions at its structural defects to form robust EDL.6−8 To overrule the limit (21 μFcm−2), the literature states that the minimum areal capacity is 50−70 μF/cm2 for the edge-oriented polished graphite.9,10 Electron transfer rates at edges are reported to be ∼1 × 105 times higher,11 leading to 3− 5 times12 higher capacitance than basal planes. Comparison of LRGONR is inappropriate because if Wu et al. do not want to consider edge effects, then they should have used capacitance of reduced graphene oxide nanoribbon (RGONR; with few holes/edges) as ∼230 Fg1− (Figure S9a in the article) in calculations instead ∼1042 Fg1− obtained from LRGONR. Another issue to be considered is BET surface area that differs with measurement parameters and cannot be taken as standard to estimate the capacitance. Instead, electrochemically accessible surface of LRGONR should be taken into account. High rate capability (5−700 mVs−1) shown in the article indicates high ionic accessibility in LRGONR. Therefore, we believe that the discussion of Wu et al. on BET surface area and microporosity is not valid in a 2D material with defects. The authors of the comment need to consider the origin of high capacitance as “the edge effect” rather than talking about the mass loading and calculation errors. Mass
W
© XXXX American Chemical Society
loading in the pristine article is optimum as used in other reports.8,13 In summary, Wu et al. in their comments have not considered the role of edge effects in graphene electrochemistry and capacitance. The specific capacitance values reported by Sahu et al. are highly reproducible and fall within the range described and observed for edge-enriched graphenes.5,13
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Sahu, V.; Shekhar, S.; Sharma, R. K.; Singh, G. Ultrahigh Performance Supercapacitor from Lacey Reduced Graphene Oxide Nanoribbons. ACS Appl. Mater. Interfaces 2015, 7, 3110−3116. (2) Casiraghi, C.; Hartschuh, A.; Qian, H.; Piscanec, S.; Georgi, C.; Fasoli, A.; Novoselov, K. S.; Basko, D. M.; Ferrari, A. C. Raman Spectroscopy of Graphene Edges. Nano Lett. 2009, 9, 1433−1441. (3) Xia, J.; Chen, F.; Li, J.; Tao, N. Measurement of quantum capacitance of graphene. Nat. Nanotechnol. 2009, 4, 505−509. (4) Dyatkin, B.; Gogotsi, Y. Effects of Structural Disorder and Surface Chemistry on Electric Conductivity and Capacitance of Porous Carbon Electrodes. Faraday Discuss. 2014, 172, 139−162. (5) Yuan, W.; Zhou, Y.; Li, Y.; Li, C.; Peng, H.; Zhang, J.; Liu, Z.; Dai, L.; Shi, G. The Edge- and Basal-Plane-Specific Electrochemistry of a Single-Layer Grapheme Sheet. Sci. Rep. 2013, 3, 2248. (6) Ambrosi, A.; Pumera, M. Electrochemically Exfoliated Graphene and Graphene Oxide for Energy Storage and Electrochemistry Applications. Chem. - Eur. J. 2016, 22, 153−159. (7) Ambrosi, A.; Poh, H. L.; Wang, L.; Sofer, Z.; Pumera, M. Capacitance of p- and n-Doped Graphenes is Dominated by Structural Defects Regardless of the Dopant Type. ChemSusChem 2014, 7, 1102−1106. (8) Yoo, J. J.; Balakrishnan, K.; Huang, J.; Meunier, V.; Sumpter, B. G.; Srivastava, A.; Conway, M.; Reddy, A. L. M.; Yu, J.; Vajtai, R.; Ajayan, P. M. Ultrathin Planar Graphene Supercapacitors. Nano Lett. 2011, 11, 1423−1427. (9) Randin, J. P.; Yeager, E. Differential Capacitance Study of StressAnnealed Pyrolytic Graphite Electrodes. J. Electrochem. Soc. 1971, 118, 711−714. (10) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications; Springer: New York, 1999; Chapter 9, pp 183−220. (11) Landis, E. C.; Klein, K. L.; Liao, A.; Pop, E.; Hensley, D. K.; Melechko, A. V.; Hamers, R. J. Covalent Functionalization and Electron-Transfer Properties of Vertically Aligned Carbon Nanofibers:
A
DOI: 10.1021/acsami.6b07737 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Comment
ACS Applied Materials & Interfaces The Importance of Edge-Plane Sites. Chem. Mater. 2010, 22, 2357− 2366. (12) Kim, T.; Lim, S.; Kwon, K.; Hong, S. H.; Qiao, W.; Rhee, C. K.; Yoon, S. H.; Mochida, I. Electrochemical Capacitances of WellDefined Carbon Surfaces. Langmuir 2006, 22, 9086−9088. (13) Kim, H. K.; Bak, S. M.; Lee, S. W.; Kim, M. S.; Park, B.; Lee, S. C.; Choi, Y. J.; Jun, S. C.; Han, J. T.; Nam, K. W.; Chung, K. Y.; Wang, J.; Zhou, J.; Yang, X. Q.; Roh, K. C.; Kim, K. B. Scalable Fabrication of Micron-Scale Graphene Nanomeshes for High-Performance Supercapacitor Applications. Energy Environ. Sci. 2016, 9, 1270−1281.
B
DOI: 10.1021/acsami.6b07737 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
