Tunable Sub-nanopores of Graphene Flake Interlayers with

Tunable Sub-nanopores of Graphene Flake Interlayers with Conductive Molecular Linkers for Supercapacitors Keunsik Lee,†,§ Yeoheung Yoon,§ Yunhee Cho,†,§ Sae Mi Lee,†,§ Yonghun Shin,‡,§ Hanleem Lee,‡,§ and Hyoyoung Lee*,†,‡,§ †

Department of Chemistry, ‡Department of Energy Science, and §Centre for Integrated Nanostructure Physics (CINAP), Institute of Basic Science (IBS), Sungkyunkwan University, 2066 Seoburo, Jangan-gu, Suwon, Gyeonggi-do 440-746, Republic of Korea S Supporting Information *

ABSTRACT: Although there are numerous reports of high performance supercapacitors with porous graphene, there are few reports to control the interlayer gap between graphene sheets with conductive molecular linkers (or molecular pillars) through a π-conjugated chemical carbon− carbon bond that can maintain high conductivity, which can explain the enhanced capacitive effect of supercapacitor mechanism about accessibility of electrolyte ions. For this, we designed molecularly gap-controlled reduced graphene oxides (rGOs) via diazotization of three different phenyl, biphenyl, and para-terphenyl bis-diazonium salts (BD1−3). The graphene interlayer sub-nanopores of rGO−BD1−3 are 0.49, 0.7, and 0.96 nm, respectively. Surprisingly, the rGO−BD2 0.7 nm gap shows the highest capacitance in 1 M TEABF4 having 0.68 nm size of cation and 6 M KOH having 0.6 nm size of hydrated cation. The maximum energy density and power density of the rGO−BD2 were 129.67 W h kg−1 and 30.3 kW kg−1, respectively, demonstrating clearly that the optimized sub-nanopore of the rGO−BDs corresponding to the electrolyte ion size resulted in the best capacitive performance. KEYWORDS: bis-diazonium salt, diazotization, graphene, sub-nanopore, supercapacitor decrease.16 Therefore, it is very important for the graphene electrode to have appropriate pore sizes that can allow the electrolyte ions to move freely with a maximized volume density. Recently, Y. Gogotsi’s group has reported that the performance of carbon supercapacitors with pore sizes less than 1 nm was anomalously improved.17 It is the most important issue that matching up pore size with electrolyte ion size yields high capacitive performance. Most electrolyte ion have sizes less than 1 nm. Yet, until now, even though the capacitive effect of graphene nanosheet gap size in the supercapacitor field has become one of the most important issues18,19 to overcome, there have been few attempts to control the pore size of conductive graphene sheets through a π-conjugated chemical carbon−carbon bond that can maintain high conductivity, totally different from GOs, to have an optimized sub-nanopore. A reaction between insulating GOs and pillar material yielded a microporous or mesoporous graphene oxide framework (GOF) with high SSA by covalent bonding,20−22 which have been studied using theoretical computation23,24 and experimental

G

raphene, a monolayer of carbon atoms arranged in a 2D honeycomb lattice,1 has garnered tremendous attention due to its electrical,2 optical,3 thermal,4 and mechanical5 properties. It has been used in applications including field-effect transistors (FET),6 memory devices,7 supercapacitors,8 transparent electrodes,9 and sensors.10 In particular, graphene has been explored as an electrode material for supercapacitors, due to its large specific surface area (SSA), high conductivity, and excellent electrochemical stability. For this application,8,11 reduced graphene oxide (rGO) nanosheets have been investigated because they can be cheaply produced on a large scale from graphene oxides (GOs), which are an oxidized form of graphite commonly made by the modified Hummer’s method.12 To produce good supercapacitor performance using graphene, many researchers have attempted to make highly porous graphene material by using a strong base,11 hydrogel form,13 antisolvent,8 3D graphene structure with CNTs14 or vertically aligned graphene structure15 while maintaining high conductivity. The resulting supercapacitors were reported to have large specific surface area (SSA), meso- and macroporous structures, and high capacitance values. However, if the pore sizes are too large, the volume density of the supercapcitor will © 2016 American Chemical Society

Received: April 9, 2016 Accepted: June 16, 2016 Published: June 16, 2016 6799

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ACS Nano Scheme 1. Reaction Scheme for Producing Pore-Controlled rGOs with Various Bis-diazonium Saltsa

a

(A) The graphene oxide (GO) is prepared by modified Hummer’s method. Optical image shows GO solution in water, concentration is 1 mg mL−1. (B) Perfectly dispersed rGO solution can be easily fabricated using sodium dodecylbenzenesulfonate (SDBS) and hydrazine (NH2NH2). Optical image shows rGO solution in water after 1 week. (C−E) Both bis-diazonium salt (BD) and rGO solution are just mixed to same molar ratio (0.33 mmol of BDs per 1 mL of dispersed rGO) at room temperature. The rGO−-BD1, -2, and -3 are successfully fabricated with its specific structure.

Figure 1. Transmission electron microscopy (TEM) images and scanning electron microscopy (SEM) images of (a, e) rGO, (b, f) rGO−BD1, (c, g) rGO−BD2, and (d, h) rGO−BD3, showing each chrateristic lateral distance.

applications such as gas storage25 and oxygen reduction reaction (ORR)26. Due to low conductivity of GO by oxygen functional groups, however, the GOF materials were not used for energy storage devices. Therefore, it is expected that if we can direct chemically link the graphene sheets at one end and the other end of 1,4-p-phenyl positions used as a conductive molecular linker (or molecular pillar), the interlayer distances of the graphene flakes can be controlled with the number of phenylenes while keeping the high electrical conductivity. Furthermore, the new chemically linked graphene flakes are expected to prevent restacking problems due to unique interlayer gap distances and also be highly stable materials even at very high temperature because of the graphene framework (GF), rather than GOF. To solve the issues, we carefully designed a direct connecting method for molecularly gap-controlled reduced graphene oxides (rGOs) via diazotization of three different molecular linker precursors, phenyl (BD1), biphenyl (BD2), and p-

terphenyl (BD3) bis-diazonium salts. Herein, we report the sub-nanopore tuning of graphene flake interlayers with three different molecular phenyl linkers between graphene and graphene sheets for high capacitance supercapacitors. Our results can clearly demonstrate that the optimized subnanopore of the interlayer pore-controlled graphene nanosheets corresponding to electrolyte ion size resulting in the best capacitive performance. The strategy for the synthesis of a molecularly linked interlayer pore-controlled graphene with phenyl linkers is presented in Scheme 1. First, we designed aryl bis-diazonium salts (BD) to result in cross-linking of the rGOs by chemical bridges. Aryl monodiazonium salts have been widely used to functionalize graphene27,28 since they are easily dissolved in aqueous solution. Uniquely, aryl BDs, which have two diazonium (−N2+) functional groups at the ortho- and paraphenyl positions, can directly link on the basal plane of rGO sheets to prevent reaggregation of rGO caused by strong π−π 6800

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Figure 2. Structural characterizations of rGO and rGO−BDs. (a) X-ray photoelectron microscopy (XPS) C 1s peak comparison. (b) Comparison of integrated ID/IG ratio from Raman spectra D and G peaks. (c) Pore size distribution for N2 (calculated by HK method) comparison. (d) Size tendency of lateral distance by TEM (black) and main pore width by HK method analysis (red).

RESULTS AND DISCUSSION To confirm the structure, the rGO−BDs were characterized by transmission electron microscopy (TEM). The dominant lateral distances of rGO−BD1 (Figure 1b), rGO−BD2 (Figure 1c), and rGO−BD1 (Figure 1d) are 0.49, 0.72, and 0.96 nm, respectively (in details, Figure S4, Supporting Information). All rGO−BDs in comparing with graphite for 0.34 nm have larger lateral distance;1,30 the commonly observed lateral distances of rGO layers are 0.36 nm by hydroiodic acid with acetic acid reduction12 and 0.38 nm by hydrazine reduction31 method. For the exact comparison, we also showed the TEM image of rGO in Figure 1a, for which lateral distance is 0.38 nm. Since the dominant lateral distances from stacked rGO sheets can directly affect pore distribution, the pore size of the rGO−BDs should be larger than that of the rGOs. Because the phenyl parts of BDs are connected between rGO sheets by a rigid covalent bond, the pore is maintained with each specific molecular length. The scanning electron microscopy (SEM) image of the rGO surface at high magnification is shown in Figure 1e. The rGO sheets are well stacked due to strong π−π electrostatic interactions between graphene layers, whose surface is flat as a result of the stacked rGO film. Unlike the rGO, the SEM images of rGO−BD1, rGO−BD2, and rGO−BD3 in Figure 1f−h, respectively, show rough surfaces since the pore sizes of the rGO−BDs are larger than that of the rGO, indicating prevention of the self-restacking of the rGO−BDs.

interactions. To molecularly control the pore size of rGO nanosheets, phenyl, biphenyl, and p-terphenyl were chosen as linkers, BD1, BD2, and BD3, respectively. For the synthesis of three different types of BDs (phenyl (BD1), biphenyl (BD2), and p-terphenyl (BD3)), p-phenylenediamine, benzidine, and 4,4″-diamino-p-terphenyl were used as starting materials, respectively. To effectively link between rGO layers via diazochemisty in a solution phase reaction, sodium dodecylbenzenesulfonate (SDBS) as a surfactant was used during reduction of GOs (Figure S1, Supporting Information) to rGOs to keep the stable dispersion of rGOs in aqueous phase (Figure S2, Supporting Information). The dispersed rGOs were reacted with phenyl BDs. Briefly, for example, an electron from rGO is tranferred to the carbon position of BD1 to give a BD1 radical, while the N2 molecule is released. And then the BD1 radical could make a covalent bond with a carbon atom of the rGO sheet, changing to carbon−carbon sp3 hybridization (Figure S3, Supporting Information).28,29 We used same molar ratio for each rGO−BD (0.33 mmol of BDs per 1 mL of dispersed rGO). Finally we successfully cross-linked rGOs with phenyl derivatives (phenyl for rGO−BD1, biphenyl for rGO−BD2, and p-terphenyl for rGO−BD3) and clearly characterize their structure with different interlayer gap distances. The rGO−BDs are applied on supercapacitor cells to prove their electrical properties and capacitive effect tendency with the different interlayer pore size. The bare rGO without using BD linkers is also prepared for a control experiment. 6801

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Figure 3. Electrochemical behavior of rGO and rGO−BDs in (a−c) 6 M KOH aqueous and (d−f) 1 M TEABF4 organic electrolyte. (a, d) Cyclic voltammetry (CV) curves. (b, e) Dependence of the capacitive current (extracted from the CV curves for the discharge) on the applied scan rate and linear relationship table with R2 (inset). (c, f) Galvanometric charge−discharge (GCD) curves.

disorder.33 The G band involves the in-plane bond-stretching motion of pairs of carbon sp2 atoms. This band does not require the presence of six-membered rings, so it occurs at all sp2 sites, not only those in the rings.34 There are small peaks on rGO−BD2 and rGO−BD3 at low wavenumber, which came from the vibration/twisting mode between phenyl groups used as molecular linkers. And the G band of rGO−BD3 showed a low full-width at half-maximum (fwhm), involved by revealed single molecular linkers.35,36 We could also confirm that rGO− BD3 has the longest interlayer distance based on the integrated area D to G (ID/IG) ratio, whose ratio was calculated from Lorentzian fitted curves integrated area. In physically defective graphitic materials, the integrated ID/IG ratio is dependent on the crystallite size (La) based on the following relationship: ID/ IG = C(λ)/La, where C(λ) = 11 nm for λ = 514 nm.37,38 From rGO to rGO−BD3, the integrated ID/IG ratio gradually increased (Figure 2b), indicating that rGO−BD3 has the highest integrated ID/IG ratio corresponding to the lowest La. In particular, rGO−BD3 showed the largest lateral distance (Figure 1d), which may increase the disorder of the graphene crystalline structure (AB stacking), leading to an increase of the integrated ID/IG ratio with a decrease of the La value.38 And the increase of fwhm of D band also affects the disorder of the graphene crystalline structure.38 The powder X-ray diffraction (XRD) spectroscopy can show interlayer distance (d-spacing) of materials. Figure S7 (with Lorentzian fitting lines), Supporting Information, shows XRD spectra of rGO−BDs. The centered peak of rGO appeared at higher angle than that of rGO−BDs, which indicates that the rGO−BDs have an expanded pore structure compared with the bare rGO. Also, as the phenylene length decreases from rGO−BD3 to rGO− BD1, the centered XRD peak positions were shifted toward higher angles and the resulting rGO layered pore size decreases from rGO−BD3 to rGO−BD1, which is confirmed with the

X-ray photoelectron spectroscopy (XPS) is performed to determine the structure of the rGO−BDs. Figure 2a shows the XPS C 1s peaks of rGO−BD1, rGO−BD2, and rGO−BD3. The sp2 carbon peak (C−C/CC) is observed at 284.5 eV, and this peak intensity results from rGO sheets and the phenyl parts of the BDs. The sp3 hydroxyl carbon peak (C−O), carbonyl peak (CO), and carboxyl peak (C(O)O) of the rGO−BDs appeared at 286.1, 287, and 288 eV, respectively. In detail, although the same molar ratio of BDs for each rGO− BDs was used, the XPS data show different C/O ratio depending on the used BD. The C/O ratios of rGO−BD1, rGO−BD2, and rGO−BD3 were 1.25, 1.69, and 3.43, respectively, because the rGO−BD3 structure has many sp2 carbon bonds on phenyl groups of the BD3 pillar. The reactivity of the diazonium ion depends on the substituent groups, increasing with electron withdrawing groups and decreasing with electron donating groups.32 It is assumed that the highest C/O ratio of the terphenyl linker in comparison with those of biphenyl and single phenyl may come from the effect of the delocalized sp2−sp2 π-electrons like the effect of the electron withdrawing group. So the reactivity order of BDs bisdiazonium salts to the nucleophile graphene flakes is BD3 > BD2 > BD1. Raman spectroscopy can also confirm the synthesis and structural characterization of rGO−BDs with different phenyl lengths of the BDs. In the Raman spectra of rGO and the rGO−BDs (Figure S6, Supporting Information), the D band of all rGO−BDs appeared at 1350 cm−1 and the 2D band appeared at 2700 cm−1. The 2D band broadening could be explained by the splitting of the electronic band structure of multilayer graphene.33 The G bands of rGO and rGO−BDs were observed at about 1595 cm−1. The D band is due to the A1g symmetry mode breathing vibrations of the six-membered sp2 carbon ring, and only becomes active in the presence of 6802

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Figure 4. (a) Illustration of the charging process. The adsorption of electrolyte ions on rGO occurs on only the outer sites. However, the charging of rGO−BDs can be occurred on both interlayer and outer sites. Tendency of capacitance in (b) aqueous electrolyte and (c) organic electrolyte (gravimetric capacitance, black line, and volumetric capacitance, red line). (d) Ragone plot of rGO and rGO−BDs showing energy density and power density in 1 M TEABF4 organic electrolyte and 6 M KOH aqueous electrolyte systems.

BDs. The trend and pore width agreed with lateral distances (black line) by TEM. There are some differences due to instruments used for characterization. These were ascribed to usage of reduced graphene oxide (rGO) sheets and the consequential local restacking of rGO sheets.26 Therefore, the rGO−BDs were successfully fabricated with the molecular linkers between rGO sheets and the phenyl groups of the each BD. Each specific gap size is determined based on the length of BDs, indicating easy control of graphene pore size. The performance when applied on supercapacitors using aqueous electrolyte 6 M KOH were compared with different interlayer distances of rGO−BDs. Cyclic voltammetry (CV) was first used to characterize electrical properties of rGO and rGO−BDs at a scan rate of 10 mV s−1 in Figure 3a. All CV curves maintained nearly rectangular shape even at the various scan rates (Figure S11, Supporting Information), indicating that their charge−discharge processes involved a fast charge transfer and a low resistance for the supercapacitors in materials.8,43−45 There are small redox peaks, indicating the coexistence of electric double layer capacitance and pseudocapacitance. The oxygen containing groups such as alcohol and carboxylic acid of rGO and rGO−BDs residue could react with aqueous electrolyte as CxO + H+ ↔ CxOH.46 In addition, in Figure 3b, a linear relationship exists between the discharge current densities for rGO and the rGO−BDs (extracted from CV at half of potential window for discharge curves, −0.1 V). The discharge densities of rGO and rGO−BDs electrodes exhibited linear dependence on the scan rate in range of 10−100 mV s−1. The rGO−BDs are observed to have a linear relationship with R2 = 0.999, verifying double layer capacitive material performance with the fast diffusion of electrolyte ions even at relatively

previous literature about the reduction of spacing distance by the removal of surface-located functional groups.39 As a whole, the analyses of the XRD spectra clearly confirm formation of rGO layers linked with molecular pillars.39 Figure S8, Supporting Information, shows FT-IR spectra for rGO−BDs. The aromatic CC and C−C stretching band positions can be shifted by the conjugated system. More conjugated systems can cause a shift to higher wavenumber position for the aromatic CC bond and lower wavenumber position for the aromatic C−C bond, respectively. The aromatic CC and C−C stretching band postition differences decrease from rGO− BD1 to rGO−BD3, supporting the formation of the covalent bonds between the rGO and BDs.40 The rGO−BD porosity was investigated by N2 gas adsorption−desorption measurements with advanced methods based on Brunauer−Emmett−Teller (BET) theory for specific surface area and Horvath−Kawazoe (HK) method for subnanopore (pore width rGO−BD3 (114.47 W h kg−1) > rGO− BD1 (111.61 W h kg−1) using organic electrolyte for potential window 2.5 V. Considering the selection of electrode material, rGO−BD2 also shows the best optimized sub-nanopore for application as a supercapacitor in this system.

CONCLUSION In summary, we developed a simple and efficient method for preparing the optimized sub-nanopore of interlayer porecontrolled rGOs using different lengths of phenyl linkers for supercapacitor applications. Three different kinds of rGO−BDs were successfully synthesized via diazotization of phenyl-BD on dispersed rGO sheets. As expected, rGO−BD1, rGO−BD2, and rGO−BD3 showed different interlayer pore size. Surprisingly, rGO−BD2 exhibited the highest specific capacitance in both organic and aqueous electrolytes in this study. Therefore, it is clearly determined that the sub-nanopore size of rGO−BD2 (0.7 nm) could easily accept or pass the similar ion size of TEABF4 and KOH electrolytes, allowing adsorption and desorption of electrolyte ions on the more exposed electrodes and as a result increased specific capacitance. The maximum energy density and power density of rGO−BD2 were 129.67 W h kg−1 and 30.3 kW kg−1, respectively. Our results demonstrate that the sub-nanopore tuning of graphene flake interlayers with molecular linkers to the corresponding electrolyte ions can guide applications for Li ion battery, fuel cell, gas storage, and energy conversion or harvesting systems. METHODS Preparation of GO Solution. Graphene oxide (GO) was prepared by the modified Hummer’s method.12,15 Briefly, graphite powder (6 g) was added into hot H2SO4 solution (30 mL, 80 °C) containing K2S2O8 (5 g) and P2O5 (5 g) with stirring for 5 h and then filtered and dried. The pretreated graphite powder was added into cold H2SO4 (150 mL, 0 °C) with gradual addition of KMnO4 (20 g) under stirring in an ice bath. NaNO3 (5 g) was introduced into the mixture. After 2 h stirring, triple DI water (300 mL) was added keeping the temperature at 35 °C. Finally, H2O2 (25 mL) was added to terminate the reaction in the mixture. The brown solution was diluted, dialyzed, and centrifuged to remove residual metal ions, acid, and unexploited graphite. The GO (20 mg) was dispersed in triple deionized water (20 mL) at room temperature and ultrasonicated for 1 h. Preparation of Dispersed rGO. The surfactant-dissolved dispersion of reduced graphene oxide (rGO) was based on previous research of chemically converted graphene sheets. Sodium dodecylbenzenesulfonate (SDBS, 1 wt %) surfactant was added to the GO solution (1 mg mL−1), and the mixture was homogenized for 1 h followed by ultrasonication for 1 h. The pH was adjusted to 10 using 1 M NaOH and was verified using pH paper. The prepared GO solution was reduced with 30% hydrazine hydrate (0.4 mL) at 90 °C for 24 h. The resulting solution was filtered using cotton to remove aggregates. Synthesis of Bis-daizonium Salts (BDs). For the synthesis of BD1, p-phenylenediamine (0.200 g, 1.8 mmol; Aldrich) dissolved in dry CH2Cl2, boron trifluoride etherate (2 equiv; Aldrich), and isoamyl nitrite (1.6 equiv; TCI) were 6805

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for High Performance Supercapacitors. Adv. Mater. 2013, 25, 4437− 4444. (9) Moon, I. K.; Kim, J. I.; Lee, H.; Hur, K.; Kim, W. C.; Lee, H. 2D Graphene Oxide Nanosheets as an Adhesive Over-Coating Layer for Flexible Transparent Conductive Electrodes. Sci. Rep. 2013, 3, 1112. (10) Some, S.; Xu, Y.; Kim, Y.; Yoon, Y.; Qin, H.; Kulkarni, A.; Kim, T.; Lee, H. Highly Sensitive and Selective Gas Sensor Using Hydrophilic and Hydrophobic Graphenes. Sci. Rep. 2013, 3, 1868. (11) Zhu, Y. W.; et al. Carbon-Based Supercapacitors Produced by Activation of Graphene. Science 2011, 332, 1537−1541. (12) Moon, I. K.; Lee, J.; Ruoff, R. S.; Lee, H. Reduced Graphene Oxide by Chemical Graphitization. Nat. Commun. 2010, 1, 73. (13) Nardecchia, S.; Carriazo, D.; Ferrer, M. L.; Gutierrez, M. C.; del Monte, F. Three Dimensional Macroporous Architectures and Aerogels Built of Carbon Nanotubes and/or Graphene: Synthesis and Applications. Chem. Soc. Rev. 2013, 42, 794−830. (14) Yang, X. W.; Cheng, C.; Wang, Y. F.; Qiu, L.; Li, D. LiquidMediated Dense Integration of Graphene Materials for Compact Capacitive Energy Storage. Science 2013, 341, 534−537. (15) Yoon, Y.; et al. Vertical Alignments of Graphene Sheets Spatially and Densely Piled for Fast Ion Diffusion in Compact Supercapacitors. ACS Nano 2014, 8, 4580−4590. (16) Chmiola, J.; Yushin, G.; Gogotsi, Y.; Portet, C.; Simon, P.; Taberna, P. L. Anomalous Increase in Carbon Capacitance at Pore Sizes Less Than 1 Nanometer. Science 2006, 313, 1760−1763. (17) Lukatskaya, M. R.; Mashtalir, O.; Ren, C. E.; Dall’Agnese, Y.; Rozier, P.; Taberna, P. L.; Naguib, M.; Simon, P.; Barsoum, M. W.; Gogotsi, Y. Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide. Science 2013, 341, 1502−1505. (18) Hantel, M. M.; Kaspar, T.; Nesper, R.; Wokaun, A.; Kotz, R. Partially Reduced Graphite Oxide as an Electrode Material for Electrochemical Double-Layer Capacitors. Chem. - Eur. J. 2012, 18, 9125−9136. (19) Hantel, M. M.; Nesper, R.; Wokaun, A.; Kotz, R. In-situ XRD and Dilatometry Investigation of The Formation of Pillared Graphene via Electrochemical Activation of Partially Reduced Graphite Oxide. Electrochim. Acta 2014, 134, 459−470. (20) Hung, W. S.; Tsou, C. H.; De Guzman, M.; An, Q. F.; Liu, Y. L.; Zhang, Y. M.; Hu, C. C.; Lee, K. R.; Lai, J. Y. Cross-Linking with Diamine Monomers to Prepare Composite Graphene Oxide Framework Membranes with Varying d-Spacing. Chem. Mater. 2014, 26, 2983−2990. (21) Lee, J. H.; Kang, S.; Jaworski, J.; Kwon, K. Y.; Seo, M. L.; Lee, J. Y.; Jung, J. H. Fluorescent Composite Hydrogels of Metal-Organic Frameworks and Functionalized Graphene Oxide. Chem. - Eur. J. 2012, 18, 765−769. (22) Burress, J. W.; Gadipelli, S.; Ford, J.; Simmons, J. M.; Zhou, W.; Yildirim, T. Graphene Oxide Framework Materials: Theoretical Predictions and Experimental Results. Angew. Chem., Int. Ed. 2010, 49, 8902−8904. (23) Nicolai, A.; Zhu, P.; Sumpter, B. G.; Meunier, V. Molecular Dynamics Simulations of Graphene Oxide Frameworks. J. Chem. Theory Comput. 2013, 9, 4890−4900. (24) Garberoglio, G.; Pugno, N. M.; Taioli, S. Gas Adsorption and Separation in Realistic and Idealized Frameworks of Organic Pillared Graphene: A Comparative Study. J. Phys. Chem. C 2015, 119, 1980− 1987. (25) Srinivas, G.; Burress, J. W.; Ford, J.; Yildirim, T. Porous Graphene Oxide Frameworks: Synthesis and Gas Sorption Properties. J. Mater. Chem. 2011, 21, 11323−11329. (26) Kim, T. K.; Cheon, J. Y.; Yoo, K.; Kim, J. W.; Hyun, S. M.; Shin, H. S.; Joo, S. H.; Moon, H. R. Three-dimensional Pillared Metallomacrocycle Graphene Frameworks with Tunable Micro- and Mesoporosity. J. Mater. Chem. A 2013, 1, 8432−8437. (27) Huang, P.; Jing, L.; Zhu, H. R.; Gao, X. Y. Diazonium Functionalized Graphene: Microstructure, Electric, and Magnetic Properties. Acc. Chem. Res. 2013, 46, 43−52. (28) Lomeda, J. R.; Doyle, C. D.; Kosynkin, D. V.; Hwang, W. F.; Tour, J. M. Diazonium Functionalization of Surfactant-Wrapped

obtained using a CHI660c electrochemical workstation. The specific capacitance (Cs) was calculated from the galvanometric charge−discharge curves. The following formula was used: Cs = 4I /(mΔV /Δt )

where I is the constant current, m is the total mass for both electrodes, and ΔV/Δt is calculated from the slope obtained by fitting a straight line to the discharge curve. The multiplier of 4 adjusts the specific capacitance of the cell and the combined mass of two electrodes to the capacitance and mass of a single electrode. Energy density and power density of the electrodes were calculated using the following equations: E=

1 Cs(ΔV )2 8

P=

E 3600 Δt

where Cs is the specific capacitance of the electrode, ΔV is the potential range, E is the energy density (in W h kg−1), P is the power density (in W kg−1), and Δt is the discharging time (in seconds).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02415. Characterization of used GO and rGO and electrochemical properties of each rGO and rGO−BD (PDF)

AUTHOR INFORMATION Corresponding Author

*[email protected] Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Institute for Basic Science (IBS-R011-D1). REFERENCES (1) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (2) Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. L. Ultrahigh Electron Mobility in Suspended Graphene. Solid State Commun. 2008, 146, 351−355. (3) Wang, F.; Zhang, Y. B.; Tian, C. S.; Girit, C.; Zettl, A.; Crommie, M.; Shen, Y. R. Gate-variable Optical Transitions in Graphene. Science 2008, 320, 206−209. (4) Balandin, A. A.; Ghosh, S.; Bao, W. Z.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Superior Thermal Conductivity of Single-layer Graphene. Nano Lett. 2008, 8, 902−907. (5) Lee, C.; Wei, X. D.; Kysar, J. W.; Hone, J. Measurement of The Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385−388. (6) Some, S.; Kim, J.; Lee, K.; Kulkarni, A.; Yoon, Y.; Lee, S.; Kim, T.; Lee, H. Highly Air-Stable Phosphorus-Doped n-Type Graphene Field-Effect Transistors. Adv. Mater. 2012, 24, 5481−5486. (7) Seo, S.; Yoon, Y.; Lee, J.; Park, Y.; Lee, H. Nitrogen-Doped Partially Reduced Graphene Oxide Rewritable Nonvolatile Memory. ACS Nano 2013, 7, 3607−3615. (8) Yoon, Y.; Lee, K.; Baik, C.; Yoo, H.; Min, M.; Park, Y.; Lee, S. M.; Lee, H. Anti-Solvent Derived Non-Stacked Reduced Graphene Oxide 6806

DOI: 10.1021/acsnano.6b02415 ACS Nano 2016, 10, 6799−6807

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ACS Nano Chemically Converted Graphene Sheets. J. Am. Chem. Soc. 2008, 130, 16201−16206. (29) Zhu, Y.; Higginbotham, A. L.; Tour, J. M. Covalent Functionalization of Surfactant-Wrapped Graphene Nanoribbons. Chem. Mater. 2009, 21, 5284−5291. (30) Valles, C.; Drummond, C.; Saadaoui, H.; Furtado, C. A.; He, M.; Roubeau, O.; Ortolani, L.; Monthioux, M.; Penicaud, A. Solutions of Negatively Charged Graphene Sheets and Ribbons. J. Am. Chem. Soc. 2008, 130, 15802−15804. (31) Cote, L. J.; Kim, F.; Huang, J. X. Langmuir-Blodgett Assembly of Graphite Oxide Single Layers. J. Am. Chem. Soc. 2009, 131, 1043− 1049. (32) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. M. Covalent Modification of Carbon Surfaces by Aryl Radicals Generated from The Electrochemical Reduction of Diazonium Salts. J. Am. Chem. Soc. 1997, 119, 201−207. (33) Niyogi, S.; et al. Spectroscopy of Covalently Functionalized Graphene. Nano Lett. 2010, 10, 4061−4066. (34) Ferrari, A. C.; Robertson, J. Interpretation of Raman Spectra of Disordered and Amorphous Carbon. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 14095−14107. (35) Martin-Drumel, M. A.; Pirali, O.; Falvo, C.; Parneix, P.; Gamboa, A.; Calvo, F.; Brechignac, P. Low-Energy Vibrational Spectra of Flexible Diphenyl Molecules: Biphenyl, Diphenylmethane, Bibenzyl and 2-, 3-and 4-phenyltoluene. Phys. Chem. Chem. Phys. 2014, 16, 22062−22072. (36) Liu, T. Y.; Xu, S. N.; Sun, C. L.; Zhou, M. Raman Spectroscopic Studies on p-terphenyl under High Pressure. Chem. Phys. Lett. 2014, 615, 1−5. (37) Tan, P. H.; Dimovski, S.; Gogotsi, Y. Raman Scattering of Nonplanar Graphite: Arched Edges, Polyhedral Crystals, Whiskers and Cones. Philos. Trans. R. Soc., A 2004, 362, 2289−2310. (38) Ferrari, A. C. Raman Spectroscopy of Graphene and Graphite: Disorder, Electron Phonon Coupling, Doping and Nonadiabatic Effects. Solid State Commun. 2007, 143, 47−57. (39) Yuan, K.; et al. Straightforward Generation of Pillared, Microporous Graphene Frameworks for Use in Supercapacitors. Adv. Mater. 2015, 27, 6714−6721. (40) Yang, C. J.; Jenekhe, S. A. Conjugated Aromatic Poly(azomethines). 1. Characterization of Structure, Electronic Spectra, and Processing of Thin Films from Soluble Complexes. Chem. Mater. 1991, 3, 878−887. (41) Lee, K.; Kim, D.; Yoon, Y.; Yang, J.; Yun, H. G.; You, I. K.; Lee, H. Fast Diffusion Supercapacitors via an Ultra-high Pore Volume of Crumpled 3D Structure Reduced Graphene Oxide Activation. RSC Adv. 2015, 5, 60914−60919. (42) Dombrowski, R. J.; Lastoskie, C. M. A Two-stage HorvathKawazoe Adsorption Model for Pore Size Distribution Analysis. Stud. Surf. Sci. Catal. 2002, 144, 99−106. (43) Beidaghi, M.; Wang, C. L. Micro-Supercapacitors Based on Interdigital Electrodes of Reduced Graphene Oxide and Carbon Nanotube Composites with Ultrahigh Power Handling Performance. Adv. Funct. Mater. 2012, 22, 4501−4510. (44) Pech, D.; Brunet, M.; Durou, H.; Huang, P. H.; Mochalin, V.; Gogotsi, Y.; Taberna, P. L.; Simon, P. Ultrahigh-power Micrometresized Supercapacitors Based on Onion-like Carbon. Nat. Nanotechnol. 2010, 5, 651−654. (45) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845−854. (46) Liu, F.; Xue, D. F. An Electrochemical Route to Quantitative Oxidation of Graphene Frameworks with Controllable C/O Ratios and Added Pseudocapacitances. Chem. - Eur. J. 2013, 19, 10716− 10722. (47) Lin, J.; Zhang, C. G.; Yan, Z.; Zhu, Y.; Peng, Z. W.; Hauge, R. H.; Natelson, D.; Tour, J. M. 3-Dimensional Graphene Carbon Nanotube Carpet-Based Microsupercapacitors with High Electrochemical Performance. Nano Lett. 2013, 13, 72−78. (48) Yang, S. B.; Wu, X. L.; Chen, C. L.; Dong, H. L.; Hu, W. P.; Wang, X. K. Spherical α-Ni(OH)2 Nanoarchitecture Grown on

Graphene as Advanced Electrochemical Pseudocapacitor Materials. Chem. Commun. 2012, 48, 2773−2775. (49) Portet, C.; Taberna, P. L.; Simon, P.; Laberty-Robert, C. Modification of Al Current Collector Surface by Sol-gel Deposit for Carbon-carbon Supercapacitor Applications. Electrochim. Acta 2004, 49, 905−912. (50) Shi, W. H.; et al. Achieving High Specific Charge Capacitances in Fe3O4/Reduced Graphene Oxide Nanocomposites. J. Mater. Chem. 2011, 21, 3422−3427. (51) Perez, C. R.; Yeon, S. H.; Segalini, J.; Presser, V.; Taberna, P. L.; Simon, P.; Gogotsi, Y. Structure and Electrochemical Performance of Carbide-Derived Carbon Nanopowders. Adv. Funct. Mater. 2013, 23, 1081−1089. (52) Liu, W. W.; Feng, Y. Q.; Yan, X. B.; Chen, J. T.; Xue, Q. J. Superior Micro-Supercapacitors Based on Graphene Quantum Dots. Adv. Funct. Mater. 2013, 23, 4111−4122. (53) Richey, F. W.; Dyatkin, B.; Gogotsi, Y.; Elabd, Y. A. Ion Dynamics in Porous Carbon Electrodes in Supercapacitors Using in situ Infrared Spectroelectrochemistry. J. Am. Chem. Soc. 2013, 135, 12818−12826. (54) Chmiola, J.; Largeot, C.; Taberna, P. L.; Simon, P.; Gogotsi, Y. Desolvation of Ions in Subnanometer Pores and Its Effect on Capacitance and Double-layer Theory. Angew. Chem., Int. Ed. 2008, 47, 3392−3395. (55) Ng, P. L.; Jamaludin, A.; Alias, Y.; Basirun, W. J.; Ahmad, Z. A.; Mohamad, A. A. Effect of KOH Concentration in The Gel Polymer Electrolyte for Direct Borohydride Fuel Cell. J. Appl. Polym. Sci. 2012, 123, 2662−2666.

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DOI: 10.1021/acsnano.6b02415 ACS Nano 2016, 10, 6799−6807