ARTICLE pubs.acs.org/JPCC
Functionalized Graphene-Based Nanocomposites for Supercapacitor Application Ashish Kumar Mishra and Sundara Ramaprabhu* Alternative Energy and Nanotechnology Laboratory (AENL), Nano Functional Materials Technology Centre (NFMTC), Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India ABSTRACT: A modern technological society demands the use and storage of energy on a large scale. In this regard, the development of high performance supercapacitors is the focus of current scientific research. Graphene, due to its excellent properties, has attracted attention for supercapacitor applications. In the present work, graphene is synthesized via hydrogen-induced exfoliation and is further functionalized to decorate with metal oxide (RuO2, TiO2, and Fe3O4) nanoparticles and polyaniline using the chemical route. Materials are characterized by electron microscopy, X-ray diffraction, Fourier transform infrared, and Raman spectroscopy techniques. Electrochemical performance of as-prepared graphene (HEG), functionalized graphene (f-HEG), RuO2-f-HEG, TiO2-f-HEG, Fe3O4-f-HEG, and PANI-f-HEG (PANI = polyaniline) nanocomposites is examined using cyclic voltammetry and galvanostatic chargedischarge techniques for supercapacitor applications. A maximum specific capacitance of 80, 125, 265, 60, 180, and 375 F/g for HEG, f-HEG, RuO2-f-HEG, TiO2-fHEG, Fe3O4-f-HEG, and PANI-f-HEG nanocomposites, respectively, is obtained with 1 M H2SO4 as the electrolyte at the voltage sweep rate of 10 mV/s. The specific capacitance for each nanocomposites sustains up to 85% even at higher voltage sweep rate of 100 mV/s. A simple and cost-effective preparation technique of graphene and its nanocomposites with good capacitive behavior encourages its commercial use.
1. INTRODUCTION The increasing popularity of various portable electronic devices and motor vehicles has increased the demand of energy storage devices. In this regard, different batteries and high performance capacitors are the focus of the scientific community. Supercapacitors, also called electrochemical capacitors or ultracapacitors, are able to provide a huge amount of energy in a short period of time, making them indispensable for surgepower delivery.1,2 Supercapacitors are mostly used as complementary devices to batteries and fuel cells which have high energy densities but cannot supply a high power during short time scale. Electrochemical capacitors have been considered as promising high-power sources for digital communication devices and electric vehicles. Depending on the charge-storage mechanism, they are basically classified into two types: electric double-layer capacitors (EDLCs) based on carbon electrodes3,4 and pseudocapacitors with certain metal oxides58 or conducting polymers as electrode materials.9 While the storage mechanism in carbon-based EDLCs is through electrostatic forces, fast Faradaic redox reactions are responsible for the chargestorage mechanism in pseudocapacitors.10 Transition metal oxides have been explored as potential electrode materials for use in supercapacitors; their charge storage mechanisms are based predominantly on pseudocapacitance. RuO2 has been found to have high capacitance due to redox transitions that even penetrates into the bulk of the material; however, the cost of Ru is one of the concerns for r 2011 American Chemical Society
commercial acceptance.1114 The present trend in the ongoing research on supercapacitors is to develop economical electrode materials with a high capacity of charge storage and energy density. Cheap metal oxides with comparable characteristics are being investigated, for example, oxides of Ni, Co, In, Sn, Fe, Mn, and so forth, and conducting polymers are another class of material under investigation due to their excellent electrochemical properties and low cost. Among the various conducting polymers, polyaniline (PANI) has been studied extensively because of its cost-effective and easy synthesis procedure and has good environmental stability, redox reversibility, and electrical conductivity.9,10 However, chemically prepared nanostructured conducting polymers are usually powdery and insulating in their dedoped states.15 Carbon-based nanomaterials having a high surface area and good electrical conductivity has been attracted the attention of scientific community for different applications. These carbonbased nanomaterials (activated carbon, carbon nanotubes, and graphene) have been used as substrate for metal oxide nanoparticles for supercapacitor applications.1625 These conducting carbon materials provide a fast electron transfer rate during Faradaic charge transfer reactions and hence enhance the capacitance. Additionally, these carbon nanomaterials provide Received: February 20, 2011 Revised: June 10, 2011 Published: June 21, 2011 14006
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The Journal of Physical Chemistry C the platform for the decoration of metal oxide nanoparticles to avoid their agglomeration, hence more utilization of nanoparticles. Polymer-carbon nanocomposites provide the solution for the insulating nature of conducting polymers at dedoped states by using carbon nanomaterials as substrate to grow nanostructured polymers.26 The high cost of carbon nanotubes limits their use at a large scale for commercial use. Large-scale production of high quality graphene is still a challenge, which again limits the development of commercial graphene based supercapacitor. Recently, a new method for production of graphene via hydrogen exfoliation at a relatively lower temperature (200 °C) was developed by our group.27 Herein, we demonstrate the use of hydrogen-exfoliated graphene as a supercapacitor electrode. The functionalization of graphene enhances the capacitance as well as provides anchoring sites for decoration of metal oxide nanoparticles and conducting polymer. Metal oxide (RuO2, TiO2, and Fe3O4) nanoparticles decorated over functionalized graphene nanocomposites and PANI coated functionalized graphene were also tested for supercapacitor applications. The use of graphene as a support for metal oxide nanoparticles and PANI avoids the agglomeration of nanoparticles to achieve more utilization of metal oxide characteristics and growth of nanostructured PANI. This helps in achieving a high value of capacitance. The capacitive behavior of each nanocomposite was tested by cyclic voltammetry, galvanostatic charge discharge and impedance spectroscopy techniques.
2. EXPERIMENTAL SECTION 2.1. Preparation of Graphene and Coating of Metal Oxide Nanoparticles/Polymer over Functionalized Graphene Surface. To prepare graphene sheets, graphitic oxide was prepared
first by oxidation of pure graphite using Hummers' method. In this method, the oxidation of graphite to graphitic oxide is accomplished by treating graphite with essentially a water-free mixture of concentrated sulfuric acid, sodium nitrate and potassium permanganate.28 This graphitic oxide was further thermally exfoliated at 200 °C under hydrogen atmosphere. This thermal shock under the presence of hydrogen gas leads to formation of graphene sheets.27 These hydrogen-exfoliated graphene sheets (HEG) were further treated with concentrated HNO3, which introduces hydrophilic functional groups (COOH, CdO, and OH) at the surface of HEG.29 The functionalized graphene sheets (f-HEG) were further washed several times with water to achieve pH = 7 followed by drying. These hydrophilic functional groups act as anchoring sites for metal oxide nanoparticles. RuO2 nanoparticles were decorated over f-HEG using a chemical route. In this method, f-HEG was dispersed in a solution of 1:1 volume ratio of isopropanol and deionized water by ultrasonic agitation. RuCl3 3 3H2O was used as a precursor salt and reduced slowly by dropwise addition of 1 M NaOH/0.1 M NaBH4 in the solution of f-HEG. The product was further filtered and washed with deionized water several times to remove excess chloride ions and to neutralize. This product was further dried at 100 °C in vacuum oven for 12 h. The dried product was Ru nanoparticles dispersed over the surface of f-HEG. The dried sample was further calcined at 350 °C in air atmosphere for 2 h to obtain crystalline RuO2-f-HEG nanocomposite. The calcination of Ru dispersed f-HEG was performed using a furnace to achieve the proper phase of RuO 2.21 TiO2 nanoparticles were decorated over f-HEG by a solgel technique. In this technique, a solgel solution was prepared
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using titanium tetra isopropoxide [Ti(OPri)4], isopropanol [IPA], nitric acid [HNO3], and distilled water [H2O] in the weight ratio of 1:10:1:0.2, respectively. This solgel solution was refluxed at 80 °C for 2 h using magnetic stirrer. An appropriate amount of f-HEG was mixed in this solution and was stirred about 12 h, which leads to the decoration of TiO2 nanoparticles over f-HEG surface. Further this solution was filtered and dried in oven at 80 °C followed by thermal treatment in air atmosphere at 450 °C for 1 h to achieve uniform TiO2 phase. The as-prepared sample was the TiO2-f-HEG nanocomposite.30 The decoration of Fe3O4 nanoparticles over the graphene (HEG) surface was done by a chemical technique. Functionalized graphene (f-HEG) was suspended in deionized water by ultrasonication method. Functional groups at the surface of graphene provide hydrophilic nature to graphene, which leads to easy dispersion of graphene in water. FeCl3 3 6H2O and FeSO4 3 7H2O (Acros Organics) were dissolved in deionized water in the stoichiometric ratio of 3:2. This solution was heated up to 90 °C. Ammonia solution (NH4OH-25%) and HEG dispersed solution, in the volumetric ratio of 1:5, were added in the above solution. This mixture solution was stirred at 90 °C for 30 min and then cooled to the room temperature. The black precipitate was collected by filtration and was neutralized with water.31 The obtained black precipitate was Fe3O4-f-HEG nanocomposite. The loading of metal oxides in corresponding nanocomposites was maintained about 25 wt %. The PANI-f-HEG nanocomposite was synthesized by polycondensation of aniline by K2Cr2O7 in solution of 1 M HCl. The nanocomposite material was then filtered and washed with large amount of water and subsequently with ethanol to remove the residual oxidant. Finally, all composites were washed with acetone and dried at 60 °C.20 2.2. Preparation of Supercapacitor Electrodes and Setup. Supercapacitor electrodes were prepared by coating nanocomposites on carbon paper (SGL, Germany) using a simple brush coating technique. About 5 mg of the desired electrode material (HEG, f-HEG, RuO2-f-HEG, TiO2-f-HEG, Fe3O4-f-HEG, and PANI-f-HEG) was dispersed in isopropanol (5 mL) with 10 μL of 5% nafion solution as a binder. The isopropanol and nafion solutions used were of analytical grade. The size of each electrode was maintained at 2 cm 2 cm for each material. Coated carbon paper was heated in vacuum oven at 150 °C for 12 h to reduce the effect of the binder. Carbon paper was weighed before and after material coating to get the correct value of loading of sample at each electrode. Nearly 1 mg of nanocomposite was observed at each electrode. The supercapacitor setup for testing its capacitance consists of two electrodes, a separator, an electrolyte, and two current collectors. Carbon-based material coated carbon paper was used as an electrode, and polypropylene membrane was used as separator between the two electrodes; 1 M H2SO4 was used as an electrolyte, and stainless steel sheets were used as current collectors. A separator rinsed with electrolyte was pressed between two electrodes. This assembly was further sandwiched between current collectors along with the Perspex sheet. Fe3O4-f-HEG nanocomposite electrodes were also tested with 1 M Na2SO4 as an electrolyte. 2.3. Characterization Techniques. HEG, RuO2-f-HEG, and TiO2-f-HEG nanocomposite were characterized by different microscopy and spectroscopy techniques. A structural analysis of metal oxide-graphene nanocomposites was performed using an X'Pert Pro PANalytical X-ray diffractometer. 14007
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Figure 1. TEM images of HEG (a), RuO2-f-HEG (b), TiO2-f-HEG (c), Fe3O4-f-HEG (d), and PANI-f-HEG (e) nanocomposites, respectively.
Figure 3. Raman spectrum of f-HEG and PANI-f-HEG. Figure 2. XRD pattern of RuO2-f-HEG, TiO2-f-HEG, and Fe3O4-fHEG nanocomposites.
The morphological analysis was done using a Philips JEOL CM12 transmission electron microscope (TEM). Raman analysis was performed by using HORIBA Jobin Yvon HR800UV confocal Raman spectrometer. A Fourier transform infrared (FTIR) study was also carried out to examine the vibrational characteristics of graphene and polymer-graphene nanocomposite. Capacitive behavior was studied using a CH instrument (CHI608C).
3. RESULTS AND DISCUSSION 3.1. Morphological Studies. Figure 1ae shows the morphological structure of HEG, RuO2-f-HEG, TiO2-f-HEG, Fe3O4f-HEG, and PANI-f-HEG nanocomposites, respectively. The TEM image of HEG (Figure-1a) clearly suggests the disorder induced by exfoliation in a graphite structure resulting in the form of sheets. The rapid removal of intercalated oxygen atoms and other functional groups in graphitic oxide during exfoliation results in a wrinkled structure of graphene sheets. TEM images of RuO2-f-HEG (Figure 1b), TiO2-f-HEG (Figure 1c), and Fe3O4f-HEG (Figure 1d) nanocomposites exhibit a uniform decoration
of corresponding metal oxide nanoparticles over the f-HEG surface. Particle sizes of metal oxides were found to be in the range of 815 nm for each nanocomposite. The TEM image of the PANI-f-HEG nanocomposite shows the uniform coating of polyaniline over the f-HEG surface. 3.2. X-ray Diffraction (XRD) Analysis. XRD patterns of RuO2-f-HEG, TiO2-f-HEG, and Fe3O4-f-HEG nanocomposites are shown in Figure 2. The XRD pattern of RuO2-f-HEG nanocomposites exhibits the peak of RuO2 at 2θ values of 28.2°, 35.4°, 40.3°, 54.5°, 58.5°, 69.5°, and 78.5°. The XRD pattern of the RuO2 -f-HEG nanocomposite suggests the formation of a tetragonal structure of RuO2 nanoparticles as shown in Figure 2.21 The XRD pattern of the TiO2-f-HEG nanocomposite (Figure 2) exhibits the peak of TiO2 at 2θ values of 25.4°, 38.3°, 48.4°, 54.7°, and 63.2°. The XRD pattern of the TiO2-f-HEG nanocomposite suggests the formation of a tetragonal structure of the anatase phase of TiO2 nanoparticles.30 The XRD pattern of the Fe3O4-f-HEG nanocomposite exhibits the peak of magnetite at 2θ values of 30.5°, 36°, 43.5°, 53.8°, 57.5°, and 63.1°. These peaks correspond to the face centered cubic structure of magnetite nanoparticles.31 Any significant peak corresponding to the graphitic structure could not be observed in any nanocomposite because of high degree of 14008
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Figure 4. FTIR spectrum of f-HEG and PANI-f-HEG.
disorderness created in graphite to form graphene sheets through the exfoliation technique. 3.3. Raman Spectrum Analysis. Raman spectroscopy was performed to analyze the vibrational characteristics of f-HEG and PANI-f-HEG. Figure 3 shows two peaks corresponding to the D-band (1350.6 cm1) and G-band (1595.4 cm1) for f-HEG. The D-band corresponds to the defects induced in the graphitic structure and functional groups attached to the surface of graphene sheets, while the G-band corresponds to the in-plane vibrations of the graphitic structure.32 The Raman lines of the PANI-f-HEG nanocomposite appear at 423.5, 529, 771.8, 1166.1, 1212.5, 1329.6, 1403.9, 1485.8, and 1578.9 cm1, corresponding to the phenazine-like segment, out-of-plane CH deformation of quinonoid ring, CH bending in quinonoid ring, CN stretching, CH bending in benzenoid ring, semiquinone radical, and CdN, CdC, and C—C stretchings, respectively.33,34 The appearance of the above peaks suggests the formation of emeraldine salt type polyaniline over the surface of f-HEG. 3.4. FTIR Spectrum Analysis. FTIR spectra of f-HEG and PANI-f-HEG are shown in Figure 4. The band corresponds to the hydroxyl group (OH stretching, 3430 cm1) is quite prominent as compared to the insignificant ratios of antisymmetric and symmetric dCH2 vibrations (2923 and 2853 cm1) for f-HEG. The peak at 1627 cm1 may be attributed to O—H bending vibrations. Intense peaks correspond to the CdO and C—O stretching vibrations of COOH groups at 1741 and 1048 cm1, respectively, can also be observed for f-HEG.32 The FTIR spectrum of PANI-f-HEG shows the low intensity of antisymmetric and symmetric =CH2 vibrations (2923 and 2853 cm1) of graphene along with the hydroxyl group (OH stretching, 3434 cm1). In addition, peaks at 505, 797, 1294, 1484, and 1563 cm1 were also observed for PANI-f-HEG nanocomposite corresponding to deformation of benzenoid ring (B), C—C stretching of the quinonoid, C—N stretching of secondary aromatic amines, benzenoid ring stretching, and quinonoid (Q) ring stretching, respectively. The peak at 1241 cm1 may correspond to the C—NH+ stretching (characteristic of polaron form of PANI emeraldine salt), while the peak at 1126 cm1 may be assigned to the stretching vibration of —NH+d (in the B—NH+dQ segment) in the bipolaron form of a PANI emeraldine salt.33,34 These peaks clearly suggest the formation of emeraldine salt type polyaniline in PANI-f-HEG nanocomposite.
Figure 5. (a) Cyclic voltammetry study of HEG, f-HEG, RuO2-f-HEG, and TiO2-f-HEG nanocomposite with the voltage sweep rate of 10 mV/s and 1 M H2SO4 as an electrolyte. (b) Cyclic voltammetry study of Fe3O4-f-HEG with 1 M Na2SO4 as an electrolyte and PANI-f-HEG nanocomposite with 1 M H2SO4 as an electrolyte at the voltage sweep rate of 10 mV/s. (c) Variation of specific capacitance with voltage sweep rate for HEG, f-HEG, RuO2-f-HEG, TiO2-f-HEG, Fe3O4-f-HEG, and PANI-f-HEG nanocomposite.
3.5. Electrochemical Studies for Capacitance Measurement. 3.5.1. Cyclic Voltammetry Studies. Cyclic voltammetry
studies were performed to analyze the capacitance behavior of HEG, f-HEG, RuO2-f-HEG, TiO2-f-HEG, Fe3O4-f-HEG, and PANI-f-HEG nanocomposites. Figure 5a shows the cyclic voltammetry curves with the voltage sweep rate of 10 mV/s for each of the three samples studied. The cyclic voltammetry study for HEG, f-HEG, RuO2-f-HEG, TiO2-f-HEG, Fe3O4-f-HEG, and PANI-f-HEG nanocomposites was carried with a two-electrode 14009
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Table 1. Specific Capacitance of HEG, f-HEG, RuO2-f-HEG, TiO2-f-HEG, Fe3O4-f-HEG, and PANI-f-HEG Nanocomposites Using Cyclic Voltammetry with Different Voltage Sweep Rates and Using the Galvanostatic Charge-Discharge Technique specific capacitance (F/g) (cyclic voltammetry) voltage scan rate
voltage scan rate
voltage scan rate
voltage scan rate
specific capacitance (F/g) with current
10 mV/s
20 mV/s
50 mV/s
100 mV/s
densities (galvanostatic)
Sample HEG f-HEG RuO2- f-HEG
80
77
70
65
125 265
122 245
112 225
105 215
TiO2-f-HEG
60
58
53
40
Fe3O4-f-HEG
180
173
165
160
Fe3O4-f-HEG (1 M Na2SO4) PANI-f-HEG
65
62
58
55
375
360
340
320
supercapacitor setup and 1 M H2SO4 as an electrolyte. In addition, the cyclic voltammetry study of a Fe3O4-f-HEG nanocomposite was also performed with 1 M Na2SO4 as an electrolyte due to better capacitance characteristics of Fe3O4-f-HEG nanocomposites reported in alkaline electrolytes.35 Initially, 500 cycles were performed to stabilize the performance of capacitance for each studied material. Specific capacitance (Cs) was measured using the following equation Cs ¼ 2
I m
dV dt
ð1Þ
where I is the average current, dV/dt is the potential sweep rate, and m is the mass of nanocomposite at each electrode.10 A factor of 2 is incorporated due to the series capacitance formed in a twoelectrode system. Cyclic voltammetry curves for HEG, f-HEG, RuO2-f-HEG, TiO2-f-HEG, and PANI-f-HEG nanocomposites were found to be almost rectangular in shape with 1 M H2SO4 as an electrolyte, suggesting the capacitive behavior of each nanocomposite. While the cyclic voltammetry curve for Fe3O4-f-HEG nanocomposites shows a better rectangular shape for 1 M Na2SO4 as an electrolyte compared to 1 M H2SO4. Specific capacitance values with 1 M H2SO4, calculated using equation 1, were found to be 80, 125, 265, 60, 180, and 375 F/g for HEG, f-HEG, RuO2-f-HEG, TiO2-f-HEG, Fe3O4-f-HEG, and PANI-f-HEG nanocomposites, respectively, at a voltage sweep rate of 10 mV/s (Figure 5a,5b, Table 1). The Fe3O4-f-HEG nanocomposite shows a rectangular cyclic voltammetry curve and specific capacitance of 65 F/g with 1 M Na2SO4 as an electrolyte at voltage sweep rate of 10 mV/s (Figure 5b, Table 1). It suggests the better capacitive characteristic of the Fe3O4-f-HEG nanocomposite with alkaline based electrolyte, as also suggested in earlier reports with Fe3O4-based nanocomposites. This value is found to be higher than other reported values of the Fe3O4-based nanocomposite with a sulfate and sulfite-based electrolyte. A large enhancement in capacitance (about 56%) was observed by the functionalization of graphene. This large enhancement in capacitance may be attributed to the possible pseudo capacitance due to the availability of oxygencontaining functional groups and possible modification of pores in graphene. An enhancement of more than two times in specific capacitance of the RuO2-f-HEG nanocomposite and 43% for the Fe3O4-f-HEG nanocomposite compared to the f-HEG was observed. This large enhancement in capacitance may be attributed to the additional Faradaic charge transfer involved due to
62 (8 A/g) 110 (10 A/g) 220 (10 A/g) 38 (6 A/g) 140 (10 A/g) 45 (4 A/g) 355 (10 A/g)
the presence of metal oxide nanoparticles along with a doublelayer formation at the electrodeelectrolyte interface. The specific capacitance of TiO2-f-HEG shows a lower value compared to f-HEG, suggesting the absence of Faradaic reactions associated with TiO2 nanoparticles and the lower conductivity of TiO2-f-HEG nanocomposite due to the TiO2 nanoparticles. The RuO2-f-HEG nanocomposite shows the best performance among all of the studied metal oxide-based nanocomposites, which may be attributed to the better electrical conductivity and fast Faradaic reactions for RuO2 nanoparticles compared to other metal oxides. The PANI-f-HEG nanocomposite shows the highest value of capacitance (375 F/g) among the studied nano composites. The high capacitive behavior of the PANI-f-HEG nanocomposite can be associated with redox reactions involved for the conducting polymer due to the presence continuous oxidation states. The performance of each nanocomposite using a cyclic voltammetry study is summarized in Table 1. In addition, the variation of capacitance with different voltage sweep rates was examined for each sample. Cyclic voltammetry with different sweep rates of 10, 20, 50, and 100 mV/s was performed for each sample. Figure 5c shows the variation of specific capacitance with the voltage sweep rate. At a higher voltage sweep rate, the specific capacitance decreases slightly for each sample. About 1016% of reduction in specific capacitance was observed for each studied nanocomposite, suggesting the high degree of sustainability of capacitance even at a higher voltage sweep rate. 3.5.2. Galvanostatic ChargeDischarge Study. A galvanostatic chargedischarge study was performed to understand the sustainability of capacitive behavior of supercapacitors with each of the studied sample. The specific capacitance (Cs) was measured by a charge discharge method using the following equation Cs ¼ 2
I ðΔtÞ m ðΔV Þ
ð2Þ
where I is the discharge current, m is the mass of each electrode, and Δt is the time required for discharging the capacitor during the voltage drop of ΔV.10 Figure 6 shows the galvanostatic chargedischarge curves for each of the studied nanocomposites. Figure-6a shows the chargedischarge study of HEG, f-HEG, RuO2-f-HEG, and TiO2-f-HEG with almost triangular shape of the curves, suggesting the high degree of symmetry in charge and discharge leading to the ideal capacitive behavior. Specific capacitances 14010
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Figure 7. Electrochemical impedance spectroscopy study of HEG, f-HEG, RuO2-f-HEG, TiO2-f-HEG, and PANI-f-HEG nanocomposite with 1 M H2SO4 as an electrolyte and of Fe3O4-f-HEG with 1 M Na2SO4 as an electrolyte in the frequency range 100 kHz to 0.005 Hz.
Figure 6. (a) Galvanostatic chargedischarge study of HEG, f-HEG, RuO2-f-HEG, and TiO2-f-HEG with 1 M H2SO4 as an electrolyte. (b) Galvanostatic chargedischarge study of Fe3O4-f-HEG with 1 M Na2SO4 as electrolyte and PANI-f-HEG nanocomposite with 1 M H2SO4 as an electrolyte.
using the chargedischarge technique at different current densities were found to be 62 (at 8 A/g), 110 (at 10 A/g), 220 (at 10 A/g), 38 (at 6 A/g), and 140 F/g (at 10 A/g) for HEG, f-HEG, RuO2-f-HEG, TiO2-f-HEG, and Fe3O4-f-HEG nanocomposites, respectively, with 1 M H2SO4 as an electrolyte. The RuO2-f-HEG nanocomposite shows a higher capacitance even at a high value of current density (10 A/g) compared to other metal oxide-based nanocomposites. This again can be attributed to the better electrical conductivity and fast and reversible Faradaic reactions for RuO2 nanoparticles. Figure 6b shows the chargedischarge study of Fe3O4-f-HEG and PANI-f-HEG nanocomposites. The chargedischarge curve for the Fe3O4-f-HEG nanocomposite shows a better triangular shape in 1 M Na2SO4 (with specific capacitance of 45 F/g at current density of 4 A/g) compared to 1 M H2SO4 as an electrolyte suggesting better capacitive behavior of
Fe3O4-f-HEG nanocomposite for alkaline-based electrolyte. The PANI-f-HEG nanocomposite shows a specific capacitance of 355 F/g at the current density of 10 A/g with a high level of symmetry in the triangular chargedischarge curve. A high value of capacitance for PANI-f-HEG nanocomposite may be attributed to the availability of continuous range of states of oxidation that arise with increasing electrode potential and the reversibility of the Faradaic processes corresponding to charge withdrawal and reinjection for the conducting polymer. The performance of each nanocomposite using a chargedischarge study is summarized in Table 1. 3.5.3. Electrochemical Impedance Spectroscopy Study. Capacitive characteristics of nanocomposites were also studied using AC electrochemical impedance spectroscopy (EIS). The EIS data were analyzed using Nyquist plots. Nyquist plots show the frequency response of the supercapacitor assembly and are a plot of the imaginary component (Z00 ) of the impedance against the real component (Z0 ). Nyquist plots of different nanocomposites in the frequency range (100 kHz to 0.005 Hz) are shown in Figure 7. AC impedance spectroscopy of all nanocomposites is performed using 1 M H2SO4 as an electrolyte except for Fe3O4-fHEG nanocomposite for which 1 M Na2SO4 was used as an electrolyte. Each data point is at a different frequency with the lower left portion of the curve corresponding to the higher frequencies. The more vertical curve suggests the better capacitive behavior of supercapacitor assembly.24 Functionalized HEG shows a better vertical curve compared to HEG, suggesting the better capacitive behavior of f-HEG. This may be attributed to the possible inclusion of pseudo capacitance. Among all studied metal oxide nanocomposites, the RuO2-f-HEG nanocomposite shows the best vertical curve suggesting its high capacitive behavior. This may be attributed to the highly reversible redox reactions associated with RuO2 nanoparticles along with better electrical conductivity of RuO2. The PANI-f-HEG nanocomposite also shows good capacitive behavior similar to f-HEG. 3.6. Comparison with Other Studies. RuO2-carbon based nanocomposites have been popular as promising materials for supercapacitor electrode. Wu et al. have reported the specific capacitance of 597 F/g at low voltage scan rate of 1 mV/s with 38.3 wt % of Ru on graphene. This is equivalent to 57 wt % of RuO2 on graphene. They have shown around 250 F/g specific 14011
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Table 2. Specific Capacitance of Different Reported Composites specific capacitance sample
(F/g)
reference
RuO2-MWNTs 20 wt % RuO2
138 F/g
21
RuO2-graphene
250 F/g
25
210 F/g
36
5.1 F/g
37
anhydrous mesoporous RuO2
58 F/g
38
RuO2TiO2 nanotubes
50 F/g
39
22 wt % RuO2 RuO2TiO2 composite RuO2 nanorods over CNT
composite nanoporous RuO2 thin film
50 F/g
40
RuO2 mesoporous carbon
174 F/g
41
24 wt % RuO2 RuO2-f-HEG 25 wt % RuO2
265 F/g at 10 mV/s
Fe3O4 + 10 wt % carbon black
voltage scan rate 5.3 F/g
35
37.9 F/g
42
present work
(1 M Na2SO4 as electrolyte) Fe3O4 + activated carbon (6 M KOH as electrolyte) Fe3O4-f-HEG (1 M Na2SO4 as electrolyte)
45 F/g at 4 A/g
present
current density
work
PANI/graphene
210 F/g
43
PANI/graphene PANI/graphene
233 F/g 200 F/g at 1 A/g
44 45
current density PANI-f-HEG
355 F/g at 10 A/g
present
current density
work
capacitance with 15 wt % of Ru (i.e., ∼22 wt % of RuO2).25 In the present work, we could achieve specific capacitance of 265 F/g with 25 wt % of RuO2 in the RuO2-f-HEG nanocomposite at a higher voltage sweep rate of 10 mV/s. Liu and Zhang have demonstrated the higher specific capacitance of 210 F/g for NiO-RuO2 composite, a lower operating voltage range (00.4 V).36 Lin et al. report the maximum specific capacitance of 5.1 F/g for RuO2 nanorods over the CNT surface.37 Subramanian et al. have reported a maximum specific capacitance of 58 F/g for anhydrous mesoporous RuO2.38 Wang et al. have reported a maximum specific capacitance of 50 F/g for the symmetric supercapacitor and 46 F/g for the asymmetric supercapacitor with a RuO2TiO2 nanotube composite as electrode material.39 Patake and Lokhande have reported the specific capacitance of 50 F/g for nanoporous RuO2 thin film.40 Reddy and Ramaprabhu report the specific capacitance of 138 F/g for a RuO2-MWNTs nanocomposite even at a lower voltage scan rate.21 Jang et al. have reported the specific capacitance of 174 F/g with 24 wt % loading of RuO2 over mesoporous carbon.41 RuO2-f-HEG nanocomposites in the present work show a higher capacitance even at the higher current density (220 F/g at 10 A/g current density) compared to the above reports. High capacitance sustainability even at higher current density (10 A/g) may be due to the better adhesion between RuO2 nanoparticles and f-HEG.
Wang and Wu reported specific capacitances of 27.0, 5.7, and 5.3 F/g for Na2SO3(aq), KOH(aq), and Na2SO4(aq), respectively, for a magnetite supercapacitor with 10% carbon black as a conducting additive.35 Du et al. have reported the asymmetric capacitor with Fe3O4/activated carbon electrodes having a capacitance of 37.9 F/g in an aqueous electrolyte of 6 M KOH.42 The present study of Fe3O4-f-HEG nanocomposite shows much higher performance even at higher current density with 1 M H2SO4 (140 F/g at 10 A/g current density) and 1 M Na2SO4 (45 F/g at 4 A/g current density). The high value of capacitance in present work compared to other reported value may be attributed to the uniform dispersion of magnetite nanoparticles over the surface of HEG due to functionalization. Wu et al. report electrochemical capacitance of 210 F/g for graphene/poyaniline nanofiber composite film at a low discharge current of 0.3 A/g.43 Wang et al. report the specific capacitance of 233 F/g for graphene/polyaniline composite paper.44 Zhang et al. report the specific capacitance of 480 F/g for the polyaniline/graphene nanocomposite at a lower current density of 0.2 A/g. The specific capacitance drops drastically to 200 F/g at a 1 A/g discharge current density.45 In the present work, the PANI-f-HEG nanocomposite shows a much higher capacitance (355 F/g) even at large values of discharge current density (10 A/g). The sustainability of high capacitance for the PANI-f-HEG nanocomposite compared to other reports even at high current density values may be attributed to the better adhesion between PANI and HEG due to the functionalization of HEG and hence possibly lesser contact resistance between PANI and HEG. All above comparisons are summarized in Table 2.
4. CONCLUSIONS The present study shows that the functionalization of graphene enhances the capacitance due to the possible inclusion of pseudocapacitance associated with oxygen-containing functional groups, as well as providing anchoring sites for metal oxide nanoparticles and polymer decoration over the graphene surface. The RuO2-f-HEG nanocomposite (Cs = 220 F/g at 10 A/g discharge current density) shows a consistent high value of capacitance at even large voltage sweep rates and high current density compared to the TiO2-f-HEG and Fe3O4-f-HEG nanocomposite, which may be attributed to the better electrical conductivity and reversible Faradaic reactions for RuO2 nanoparticles compared to other metal oxide nanoparticles studied. The PANI-f-HEG nanocomposite (Cs = 355 F/g at 10 A/g discharge current density) shows the best performance as a supercapacitor electrode compared to other studied materials. This may be attributed to the reversibility of the Faradaic processes associated with a continuous range of oxidation states available for conducting polymer and better adhesion between PANI and f-HEG and hence possibly faster electron transfer during charge storage. The low cost and sustainability of high capacitance of PANI-f-HEG nanocomposite even at high discharge current (10 A/g) encourage its possible use in commercial applications. ’ AUTHOR INFORMATION Corresponding Author
*Email:
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’ ACKNOWLEDGMENT The authors acknowledge the support of IITM and DST, India. One of the authors (Ashish) is thankful to DST India for providing the financial support. ’ REFERENCES (1) Kotz, R.; Carlen, M. Electrochim. Acta 2000, 45, 2483. (2) Frackowiak, E.; Beguin, F. Carbon 2001, 39, 937. (3) Pandolfo, A. G.; Hollenkamp, A. F. J. Power Sources 2006, 157, 11. (4) Lee, Y. H.; An, K. H.; Lee, J. Y.; Lim, S. C. Encycl. Nanosci. Nanotechnol. 2004, 1, 625. (5) Liu, K. C.; Anderson, M. A. J. Electrochem. Soc. 1996, 143, 124. (6) Yoon, Y. S.; Cho, W. I.; Lim, J. H.; Choi, D. J. J. Power Sources 2001, 101, 126. (7) Conway, B. E.; Briss, V.; Wojtowicz, J. J. Power Sources 1997, 66, 1. (8) Lin, C.; Ritter, J. A.; Popov, B. N. J. Electrochem. Soc. 1998, 145, 4097. (9) Burke, A. J. Power Sources 2000, 91, 37. (10) Conway, B. E. Electrochemical Supercapacitors - Scientific fundamentals and technological applications; Kluwer Academic/Plenum Publishers, New York, 1999. (11) Toupin, M.; Brousse, T.; Belanger, D. Chem. Mater. 2004, 16, 3184. (12) Toupin, M.; Brousse, T.; Belanger, D. Chem. Mater. 2002, 14, 3946. (13) Kim, H.; Popov, B. N. J. Power Sources 2002, 104, 52. (14) Zheng, J. P.; Cyjan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699. (15) Huang, J. X.; Kaner, R. B. Chem. Commun. 2006, 367–376. (16) Portet, C.; Taberna, P. L.; Simon, P.; Flahaut, E. J. Power Sources 2005, 139, 371. (17) Beguin, F.; Szostak, K.; Lota, G.; Frackowiak, E. Adv. Mater. 2005, 17, 2380. (18) Chatterjee, A. K.; Sharon, M.; Banerjee, R.; Spallart, M. N. Electrochim. Acta 2003, 48, 3439. (19) Chen, J. H.; Li, W. Z.; Wang, D. Z.; Yang, S. X.; Wen, J. G.; Ren, Z. F. Carbon 2002, 40, 1193. (20) Reddy, A. L. M.; Amitha, F. E.; Jafri, I.; Ramaprabhu, S. Nanoscale Res. Lett. 2008, 3, 145. (21) Reddy, A. L. M.; Ramaprabhu, S. J. Phys. Chem. C 2007, 111, 7727. (22) Wang, Y.; Shi, Z.; Huang, Y.; Ma, Y.; Wang, C.; Chen, M.; Chen, Y. J. Phys. Chem. C 2009, 113, 13103. (23) Li, F.; Song, J.; Yang, H.; Gan, S.; Zhang, Q.; Han, D.; Ivaska, A.; Niu, L. Nanotechnology 2009, 20, 455602. (24) Stoller, M. D.; Park, S.; Zhu, Y.; An, J.; Ruoff, R. S. Nano Lett. 2008, 8, 3498. (25) Wu, Z. S.; Wang, D. W.; Ren, W.; Zhao, J.; Zhou, G.; Li, F.; Cheng, H. M. Adv. Funct. Mater. 2010, 20, 3595. (26) Jang, J.; Bae, J.; Choi, M.; Yoon, S. H. Carbon 2005, 43, 2730. (27) Kaniyoor, A.; Baby, T. T.; Ramaprabhu, S. J. Mater. Chem. 2010, 20, 8467. (28) Hummers, W., Jr.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (29) Kim, U. J.; Furtado, C. A.; Liu, X.; Chen, G.; Eklund, P. C. J. Am. Chem. Soc. 2005, 127, 15437. (30) Lee, T. Y.; Alegaonkar, P. S.; Yoo, J. B. Thin Solid Films 2007, 515, 5131. (31) Mishra, A. K.; Ramaprabhu, S. J. Phys. Chem. C 2010, 114, 2583. (32) Mishra, A. K.; Ramaprabhu, S. Desalination 2011, doi: 10.1016/ j.desal.2011.01.038. (33) Boyer, M. I.; Quillard, S.; Rebourt, E.; Louarn, G.; Buisson, J. P.; Monkman, A.; Lefrant, S. J. Phys. Chem. B 1998, 102, 7382.
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