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Comment on “Ultrahigh Performance Supercapacitor from Lacey Reduced Graphene Oxide Nanoribbons” Xiaozhong Wu, Wei Xing,* and Zifeng Yan* School of Science, State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, P. R. China
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supercapacitor. Thus, we do suggest that the authors characterize the micropores of LRGONR using the adsorbate of CO2 to obtain more accurate interface for the charge storage. In addition, our study also indicates that the electrolyte (1 M TEA-BF4/PC) with big ionic size could not penetrate into those ultramicropores, resulting in a lower specific capacitance for carbon material in organic electrolyte compared with that in aqueous electrolyte.10 Therefore, we suppose that the reported high specific capacitances in organic and ionic liquid electrolytes were overestimated in this paper due to their large ionic size even if considering the possible existence of such ultramicropores. According to the discussion above, electrochemically available surface with the particular counterion may be taken into account to investigate the origin of high capacitance for LRGONR. However, the actual surface area available for electrolyte ions is hard to be estimated accurately. Therefore, BET surface area detected by N2 or micropore surface area detected by CO2 could be used for reference. By comparing the size of N2 with electrolyte ions, especially for organic and ionic liquid electrolyte used by the authors, it is reasonable that the surface that is not accessible to N2 with a smaller size could also not be accessible to the electrolyte ions with larger size. We do believe that the unreported high capacitances of LRGONR in aqueous, organic and ionic liquid electrolytes may be attributed to experimental errors or calculation mistakes by the authors. As a suggestion, we recommend the authors to load more active materials during the electrode preparation process. A low loading of active materials will affect the accuracy of the estimated total mass and volume for the active materials, thus bringing about potential calculation mistakes.
ecently, Sahu et al. synthesized lacey reduced graphene oxide nanoribbons (LRGONR) through unzipping MWCNT with strong oxidants followed by KOH activation and reduction process.1 Although LRGONR possesses a Brunauer−Emmett−Teller (BET) specific surface area of 190 m2 g−1, the specific capacitances of LRGONR was reported to reach up to the abnormally high values of 1042, 1272, and 1324 F g−1 in aqueous (H2SO4), organic (TEABF4/AN) and ionic liquid (BMIMBF4) electrolytes in a three-electrode system, respectively. The corresponding areal capacitances based on BET specific surface area are calculated to be 548, 669, and 697 μF cm−2, which are far beyond the accepted value for carbonbased materials. By contrast, some other reports on similar materials, holey graphene framework, could obtain only a moderate specific capacitance, lower than 350 F g−1 and 60 μF cm−2.2−5 As is well-known, the energy-storage mechanisms of electrochemical supercapacitors contain the electric double layer (EDL) capacitance and pseudocapacitance and both mechanisms rely on the interface between electrolyte and electrode material. As a type of commonly studied electrode materials, the intrinsic areal capacitance of carbon-based materials was proposed to be 21 μF cm−2, which is also the upper limit value of EDL capacitance for carbon-based materials.6 In this case, the EDL capacitance of LRGONR with a specific surface area of 190 m2 g−1 is calculated to be ∼40 F g−1 by multiplying its BET specific surface area and areal capacitance (taking on 21 μF cm−2 in this calculation). On the other hand, it has been reported that the stored charges per atom of accessible surface of electroactive materials in EDL capacitor and pseudocapacitor are 0.17−0.20 and 2.5 electrons, respectively.7 In this paper, the residual oxygen functionalities of LRGONR are believed to contribute to the pseudocapacitance. Assuming that all of the surface oxygen species (7%) detected by X-ray photoelectron spectroscopy is exposed to the electrolyte and contribute to the pseudocapacitance, the pseudocapacitance generated from these oxygen functionalities is calculated to be 41 F g−1 at most. Therefore, the total specific capacitance incorporating EDL capacitance and pseudocapacitance should be no more than 81 F g−1. This value is much lower than the reported specific capacitance (>1000 F g−1) in this paper. Another issue should also be taken into consideration. As has been reported in literature, micropores in carbon materials play an important role for the energy storage.8 According to our previous study, some carbons with low apparent surface area (BET specific surface area determined by N2 adsorption) may contain abundant ultramicropores (
