Superior Capacitance of Functionalized Graphene - The Journal of

Mar 14, 2011 - Synergistic Amination of Graphene: Molecular Dynamics and Thermodynamics. Vitaly V. Chaban and Oleg V. Prezhdo. The Journal of Physical...
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Superior Capacitance of Functionalized Graphene Ziyin Lin,† Yan Liu,† Yagang Yao,† Owen J. Hildreth,† Zhuo Li,† Kyoungsik Moon,† and Ching-ping Wong*,†,‡ † ‡

School of Materials Science and Engineering, Georgia Institube of Technology, 771 Ferst Drive, Atlanta, Georgia, 30332, United States College of Engineering, The Chinese University of Hong Kong, Hong Kong

bS Supporting Information ABSTRACT: We report the superior capacitance of functionalized graphene prepared by controlled reduction of graphene oxide (GO). In a solvothermal method, GO dispersed in dimethylformamide (DMF) was thermally treated at a moderate temperature (150 °C), which allows a fine control of the density of functionalities. Surface functionalities on graphene would enable a high pseudocapacitance, good wetting property, and acceptable electric conductivity. A specific capacitance up to 276 F/g was achieved based on functionalized graphene at a discharge current of 0.1 A/g in a 1 M H2SO4 electrolyte, which is much higher than the benchmark material. The excellent performance of the functionalized graphene signifies the importance of controlling the surface chemistry of graphene-based materials.

1. INTRODUCTION A supercapacitor, also called electrical double layer capacitor (EDLC), is an energy storage component applicable to electric vehicles, consumer electronics, and other devices. Compared to conventional batteries, supercapacitors have advantages of high power density, long cycle life, and low maintenance, etc.1 The performance of a supercapacitor highly depends on the properties of electrode materials.2 Recent researches into electrode materials have had a special focus on graphene-based materials due to the outstanding properties of graphene including high theoretical surface area, good electrical conductivity, and open porous structure.3 However, most works on graphene-based supercapacitors only found moderate specific capacitances of ∼200 F/g.46 The unsatisfactory performance of graphene is because that EDLC strongly relies on high specific surface area between electrode/electrolyte interface, which is limited by aggregation and poor wetting between graphene and electrolytes. A potential solution for this problem is functionalization of graphene. Surface functional groups, such as carbonyl and hydroxyl, provide pseudocapacitance as a complement to EDLC.7 The hydrophilic nature of these groups improves the wetting between graphene and polar electrolytes, and also prevents aggregations of adjacent graphene sheets. However, the penalty of losing electrical conductivity arising from the disruption of conjugated system should be avoided, and the density of functionalities needs to be carefully controlled. An ideal structure of functionalized graphene (fG) has functionalities only on edges to minimize the disruption of conjugated system, as shown in Figure 1a. It is challenging to prepare fG with controlled density of functionalities using simple direct oxidation of graphene. To r 2011 American Chemical Society

introduce oxygen-containing groups, harsh conditions are usually employed due to the inert nature of graphitic structure. For example, concentrated H2SO4 and KMnO4 are used in the Hummers method;8 concentrated HNO3 and H2SO4 as well as KClO3 are used in the Staudenmaier method.9 Under these conditions, it is unlikely to finely control the density of functionalities. As an alternative way, the reduction of the graphene oxide (GO), a highly oxidized form of graphene, is more practical in controlling the density of functionalities. However, methods developed for GO reduction, such as chemical reduction,10 microwave reduction,4,11 and thermal annealing,12 all aim to achieve an efficient and complete reduction. Controllable reduction and its applications have not been explored. In this work, a mild solvothermal method was employed to reduce GO.13 Our method distinguishes from previous reported solvothermal methods,1417 as no typical reducing agent was added and a relative low temperature (150 °C) was used. The absence of typical reducing agent, such as hydrazine, allows a high retention of oxygen-containing groups as a source of pseudocapacitance. Furthermore, the kinetics of GO reduction is slow at 150 °C so that the density of functionalities can be controlled by changing the reduction time. A specific capacitance up to 276 F/g was achieved in an optimized condition. Surface functionalities on fG are proven to be critical for achieving superior supercapacitive property, which will be discussed in terms of their influence on electrical conductivity, pseudocapacitance, wetting properties, and aggregations. Received: January 23, 2011 Revised: February 21, 2011 Published: March 14, 2011 7120

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Figure 1. (a) A ideal structure of fG which has carbonyl and hydroxyl groups on the graphene edge. (b) A possible structure of fG-3h prepared by the solvothermal method. Holes exist in the basal plane in addition to carbonyl and hydroxyl groups on the edge.

2. EXPERIMENTAL SECTION 2.1. Synthesis. GO was synthesized using the Hummer method.8 The preparation of functionalized graphene (fG) was carried out using a previously described method.13 In a typical process, GO was dispersed in dimethylformamide (DMF) by sonication to form a 1 mg/mL solution. This solution was heated to 150 °C in an oil bath for different durations, producing fG with different density of functionalities. After reduction, fG was filtrated, washed with ethanol, and dried in vacuum. The synthesis of benchmark materials, hydrazine-reduced GO, was according to the procedure described in Ruoff’s work.3 2.2. Structural Characterization. Raman characterization was carried out using a LabRAM ARAMIS, Horiba Jobin Yvon with a 532-nm-wavelength laser. FTIR characterization was performed at ambient temperature with a FTIR spectrometer (Nicolet, Magna IR 560). Thermogravimetric analysis (TGA) was carried out on a thermogravimetric analyzer (TGA Q5000, TA Instruments Co.). Samples were heated at a rate of 20 °C/min in air. The X-ray photoelectron spectroscopy (XPS) was carried out with a Thermo K-Alpha XPS. 2.3. Electrochemical Characterization. The electrochemical properties of fG were tested in a three-electrode system. fG was dispersed in N-methyl-2-pyrrolidone (NMP) containing poly(vinylidene fluoride) (PVDF) as a binder. The ratio between fG and PVDF was 95:5 by weight, and the concentration of electrode materials (fG and PVDF) was 10 mg/mL. To prepare the working electrode, 2.00 μL of fG dispersion was transferred onto a glassy carbon electrode (3 mm diameter, 0.07065 cm2 area) and then dried at 85 °C. The fG loading was calculated to be 0.0190 mg. A Pt wire and Ag/AgCl electrode filled with saturated KCl aqueous solution were used as the counter electrode and reference electrode respectively. Prior to the electrochemical test, dissolved oxygen in electrolyte was removed by bubbling N2 for 10 min. Cyclic voltammetry (CV) and galvanostatic charge/discharge were measured on a Versastat 2-channel system (Princeton Applied Research). The specific capacitance is calculated according to equation C = It/V, where I is the charge/discharge current density (A/g), t is the discharge time (t), and V is the voltage (0.8 V). The reported specific capacitance and energy density are all normalized to the weight of fG.

3. RESULTS AND DISCUSSION In this report, GO is solvothermally treated for different durations to control the density of functionalities. Thus, fG sample is denoted as fG-time, where time indicates the reaction

time (h). The best capacitive performance is obtained in the case of fG-3h, which is discussed in detail here. The capacitive performance of fG-3h was characterized in a three-electrode system where a 1 M H2SO4 was used as the electrolyte. Figure 2a shows the cyclic voltammetry (CV) curves of fG-3h. At a scan rate of 10 mV/s, fG-3h shows a clear capacitive behavior with a rectangle-like CV curve. Faradic peaks are observed at ∼0.4 V of anodic scan and ∼0.35 V of cathodic scan, corresponding to redox reactions of surface functionality which provide pseudocapacitance. When increasing the scan rate, the CV curve maintains the rectangle shape at 20 mV/s and is only slightly distorted at 50 and 100 mV/s, indicating a good wetting and easy access of ions to the fG-3h surface. However, the peaks arising from the redox reactions of surface functionalities are diminished at higher scan rates, due to the relatively slow charge/discharge kinetics compared to EDLC. The specific capacitance of fG-3h is calculated from galvanostatic discharge curves, as seen in Figure 2b and Figure S1 in the Supporting Information. The charge/discharge current density varies from 0.1 to 5 A/g, and the calculated specific capacitances at different current densities are listed in Table S1 in the Supporting Information. fG-3h shows a specific capacitance up to 276 F/g at 0.1 A/g, which is still as high as 205 F/g at 5 A/g. These values are much higher than 238 F/g and 143 F/g measured for hydrazine reduced GO at 0.1 A/g and 5 A/g respectively (Figure S2 in the Supporting Information and discussion). The Ragone plot in Figure S3 in the Supporting Information shows that the specific energy of fG-3h is 20.0 and 14.5 (W h)/kg, respectively, at specific powers of 34.5 and 1.69  103 W/kg. Moreover, fG-3h shows a high cycling stability in the charge/discharge test (Figure S4 in the Supporting Information and discussion), which is impressive for functionalized carbon materials. We would like to mention that the specific capacitance of fG-3h increases after cycling test, possible because more surface functionalities are introduced during the charge/ discharge test. This interesting phenomenon was also reported by Ma et al.18 and can be used to further functionalize graphene for supercapacitor applications. A close examination of Figure 2b shows that the discharge curves obviously deviate from the ideal linear shape. Thus, fG-3h has different specific capacitances at different potentials, which are listed in Table S1 in the Supporting Information. It is found that the specific capacitance between 0 and 0.5 V is much larger than that between 0.6 and 0.8 V. This result together with CV curves suggests that EDLC is responsible for capacitance between 0.6 and 0.8 V and pseudocapacitance becomes dominating at lower potentials. When increasing the discharge current density, the specific capacitance between 0.6 and 0.8 V shows a 7121

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Figure 2. (a) CV curves of fG-3h at different scan rates; (b) galvanostatic charge/discharge curves of fG-3h at different charge/discharge currents (curves at 1, 2.5, 5 A/g are shown in Figure S1 in the Supporting Information). 1 M H2SO4 was used as electrolyte.

Figure 3. EDLC, pseudocapacitance, and overall capacitance as a function of discharge current density. Results from references are also plotted. The specific capacitance is normalized to the largest capacitance value obtained at the lowest discharge rate. Lines are for visual aids.

slower decrease than that between 0 and 0.5 V (Figure S5 in the Supporting Information), indicating a fast charge/discharge kinetics of EDLC compared to pseudocapacitance. Assuming that the capacitance of fG-3h at 0.60.8 V are pure EDLC which does not change over the whole voltage range, the dependences of overall capacitance, EDLC and pseudocapacitance on the discharge current density can be plotted, as seen in Figure 3. The capacitances decrease when the discharge current density is increased. At the largest discharge current density of 5 A/g, the capacitance retention is still as high as 74.3%, 80.5%, and 69.6%, respectively, for overall capacitance, EDLC, and pseudocapacitance. Comparing with typical pseudocapacitive materials on graphene/GO, such as conducting polymers19,20 and metal oxides,21 fG-3h shows a much better rate performance (Figure 3). Thus, it can be inferred that the charge/discharge kinetics of pseudocapacitance in fG-3h is faster than typical pseudocapacitive reactions in graphene/GO-supported conducting polymers and metal oxides. We investigated the origin of pseudocapacitance by a set of structural characterizations of fG-3h, including X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA),

Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. Figure 4a shows high-resolution C 1s XPS spectra of GO and fG-3h. The CC fraction increases from 44.0% for starting GO to 54.0% for fG-3h. A narrowing of CC band suggests a more ordered structure of fG-3h. However, the CO and CdO fractions are still as high as 28.4% and 17.6% in fG-3h, respectively, indicating a significant amount of oxygen-containing groups. Surprisingly, with many oxygen-containing groups, fG-3h still exhibits a much enhanced thermal stability, as seen in TGA results (Figure 4b). GO has a 20.5% weight loss at ∼200 °C, arising from the decomposition of liable functionalities,22 and completely decomposes at ∼650 °C. In contrast, fG-3h shows a slow and continuous weight loss before its complete decomposition at ∼720 °C. The enhanced thermal stability of fG-3h suggests that the surface functionalities are groups that have relatively high thermal stability. FTIR spectra are used to examine the nature of surface functionalities. As shown in Figure 4c, GO shows distinct peaks ∼3420 (broad), ∼1725, and ∼1625 cm1, corresponding to hydroxyls, carbonyls (carboxylic and ketone), and absorbed water, respectively.23 After the solvothermal treatment, the most significant change is that the ∼3443 cm1 peak becomes sharper and ∼1725 cm1 peak is diminished, both due to the decomposition of carboxyls. Sharp peaks at ∼1582 and ∼1380 cm1 could be attributed to unoxidized aromatic region and hydroxyls, respectively.23 Furthermore, the broad peak ∼1222 cm1 is related to phenolic, epoxy, and ketone groups.23,24 FTIR results suggest that the reduction of GO is due to the decomposition of carbonyls, mainly carboxylic groups, and the residual functionalities are mainly hydroxyl, ketone, and epoxy groups. More information is obtained from Raman spectra. As seen in Figure 4d, the ID/IG ratios of GO and fG-3h do not vary significantly, indicating that the size of the nanographite domain does not increase after reaction. This is possibly because, instead of recovering the conjugated system, thermal reduction in our method generates holes by releasing carbonyl or epoxy groups in the form of CO2 and CO. The existence of holes in fG-3h is consistent with the theoretical prediction25 and experimental observations26,27 of the atomic structures of GO and reduced GO. On the basis of these structural characterizations, we believe that surface functionalities in fG-3h are mainly hydroxyl, epoxy, and ketone groups as well as holes. Although there is no direct evidence about the distribution of oxygen-containing groups, they are most likely on 7122

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Figure 4. Structural characterizations of GO and fG-3h: (a) high-resolution C 1s XPS spectra; (b) TGA curves (ramping rate 20 °C/min, in air); (c) FTIR spectra; and (d) Raman spectra.

the edge of graphene sheet/holes after the solvothermal treatment because previous studies show that basal plane epoxies and hydroxyls are thermally unstable.25,28 A possible structure of fG3h is shown in Figure 1b, which is similar to the ideal structure sketched in Figure 1a. The presence of holes in fG-3h, as a special type of functionality, is also important because edges have higher specific capacitance than basal plane.29 A better understanding of the pseudocapacitance in fG-3h can be gained by correlating the structure of fG-3h with previous studies on activated carbon-based supercapacitors. It was found that the pseudocapacitance arises from the reversible redox reactions between oxygen-containing groups and Hþ ions. Thus, to achieve a high capacitance, it is preferable to use a H-rich H2SO4 solution as the electrolyte. When using a pH-neutral solution, the surface redox reaction is prohibited in the absence of Hþ ions. For example, no faradic reaction is observed between 0.2 and 0.4 V for fG-3h when using a 1 M Na2SO4 as the electrolyte (Figure S6 in the Supporting Information). Moreover, mainly carbonyl/hydroxyl-type groups contribute to pseudocapacitance, rather than all oxygen-containing functional groups.30 For fG-3h, a dominating fraction of surface functionalities are carbonyl and hydroxyl groups, due to their high thermal stability compared to carboxyl groups. The carboxyl groups, which sometimes lead to degradation of carbon materials, are efficiently removed during the solvothermal reaction. Therefore, a high pseudocapacitance is obtained without sacrificing the cycling stability. This selectivity is an important advantage of our solvothermal method over other methods for producing graphene as supercapacitor electrode materials. Furthermore, we would like to mention that the electrochemical behavior of fG is sensitive to dissolved oxygen in the electrolyte. As seen in Figure S7 in the Supporting Information, oxygen molecules may react with fG during discharge process, leading to Coulomb efficiencies larger than 100% and unrealistic high capacitance values. Therefore, to study the capacitive behavior

Figure 5. Specific capacitances of fG with different reaction times. Different discharge current densities were used to measure the specific capacitance as indicated in the figure. Lines are for visual aid.

of fG, it is necessary to remove oxygen carefully before electrochemical measurements. We also studied the evolution of capacitive behavior of fG when increasing the reaction time, which gives an informative clue on the dependence of capacitance on the density of functionalities. Structure characterizations (Figures S8S11 in the Supporting Information and discussions) show that solvothermal treatment can effectively remove oxygen-containing functional groups in the first 23 h. Longer reactions (>3 h), instead of further removing surface functionalities, possibly change the atomic structure of fG. By analysis of the electrochemical test results, a volcano shape relation is found between the specific capacitance of fG and the reaction time, which is shown in Figure 5. As the starting material, GO shows a 7123

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Figure 6. CV curves of fG-1h, fG-2h, fG-4h, and fG-6h at different scan rates. 1 M H2SO4 was used as electrolyte.

highly distorted CV curve (Figure S12 in the Supporting Information). The absence of capacitive behavior for GO is due to its poor electrical conductivity which limits the charge migration through the GO sheet. As a result, the EDLC and pseudocapacitance cannot be utilized and the specific capacitance of GO is negligible. At the initial stage of reduction, the specific capacitance of fG increases significantly as a result of increasing electrical conductivity. The specific capacitances of fG-1h and fG-2h are 211 and 250 F/g respectively at 0.1 A/g. However, the low electrical conductivity still limits the utilization of pseudocapacitance, leading to a significant loss of specific capacitance. For example, the CV curves of fG-1h (Figure 6) show very weak faradic peaks between 0.3 and 0.4 V, although the oxygen content of fG-1h is higher than fG-3h. In the case of fG-2h, the faradic peaks are obvious at low scan rates but become suppressed at higher scan rates, indicating a worse rate performance than fG-3h due to poor electrical conductivity. The highest capacitance is observed in the case of fG-3h, as we have discussed previously. This “sweet spot” is the spot where the capacitance loss arising from low electrical conductivity is just compensated by the capacitance gained by pseudocapacitance. When further increasing the reduction time, the specific capacitance drops slightly for fG-4h (274 F/g at 0.1 A/g), which becomes obviously smaller for fG-6h (234 F/g at 0.1 A/g). Known that the oxygen content does not differ much for fG-3h, fG-4h, and fG-6h, we believe that the wetting property and aggregations of fG are affected adversely by longer reaction time, which account for the capacitance drop. The evidence is provided by the CV measurement of fG4h and fG-6h, which are shown in Figure 6. Compared with the CV curves of fG-3h (Figure 2a), the CV curves of fG-4h and fG-6h show more distortions from ideal rectangle shape at higher scan rates, which could be attributed to poor wetting properties.

4. CONCLUSION A solvothermal method was used to prepare fG-3h, which shows a specific capacitance up to 276 F/g at 0.1 A/g in a 1 M H2SO4 electrolyte with good rate performance and cycling stability. The high capacitance is due to the surface oxygencontaining groups, mainly carbonyls and hydroxyls, which lead to large pseudocapacitacne, less aggregation, and good wetting properties. The superior capacitive performance of fG-3h demonstrates the importance of controlling the surface chemistry of graphene for supercapacitor applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. Specific capacitances of fG-3h at different dischage current densities; more electrochemical measurements of fG-3h; and structural characterizations of GO and fGs. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: (404) 894-8391. Fax: (404) 894-9140. E-mail: cp.wong@ mse.gatech.edu.

’ ACKNOWLEDGMENT We thank Dr. Albert Tamashauski of Asbury Carbon for generously providing the graphite material. ’ REFERENCES (1) Kotz, R.; Carlen, M. Electrochim. Acta 2000, 45, 2483. (2) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845. 7124

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