ARTICLE pubs.acs.org/JPCC
Synthesis and Capacitive Properties of Manganese Oxide Nanosheets Dispersed on Functionalized Graphene Sheets Jintao Zhang, Jianwen Jiang, and X. S. Zhao* Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576
bS Supporting Information ABSTRACT: Functionalized graphene was prepared by reducing functionalized graphene oxide with poly(diallyldimethylammonium chloride) (PDDA), transferring the surface charge of reduced graphene oxide (RGO) from negative to positive. A composite material of functionalized RGO with manganese dioxide (MnO2) nanosheets can be obtained by dispersing negatively charged MnO2 nanosheets on the functionalized RGO sheets via an electrostatic coprecipitation method. The structures of composites were investigated by high-resolution transmission electron microscopy (HRTEM), which indicated that the MnO2 nanosheets dispersed on functionalized RGO sheets, exhibiting a layered structure. The composite material exhibited enhanced capacitive performances than those of pure functionalized RGO and Na-typed birnessite (Na/MnO2) sheets, attributing to the synergic effect of both components. Additionally, over 89% of original capacitance was retained after 1000 cycles, indicating a good cycle stability of the composite materials.
’ INTRODUCTION Supercapacitors are energy storage devices that have attracted much attention because of a number of important features including high power density and long cycle life.1 Energy can be stored in a supercapacitor by means of either ion adsorption at the electrode/electrolyte interface (namely, electrical double-layer capacitors, EDLCs) or fast and reversible faradic reactions (namely, pseudocapacitors).2,3 Transition metal oxides, such as ruthenium oxide,4,5 manganese oxide,6 and cobalt oxide,7 are the important electrode materials for pseudocapacitors. However, several problems such as the high cost, low electrical conductivity, and poor stability limit the practical applications of metal oxides as supercapacitor electrodes. Graphene, a two-dimensional carbon material with unique mechanic and electronic properties, offers a good opportunity to fabricate graphenemetal oxide composites as electrode materials.8 Various metal oxides, such as tin oxide,9 nickel oxide,10 ruthenium oxide,11 and manganese oxide,12,13 have been anchored on graphene oxide (GO) or reduced graphene oxide (RGO) as electrode materials for supercapacitors by using chemical reactions or physical mixing. The graphenemetal oxide composites exhibited enhanced capacitive performance due to the high conductivity and high surface/volume ratio of graphene. To exploit the potential of graphene-based materials for wide applications, many efforts have been focused on functionalized RGO to improve the diversity of RGO. Covalent functionalization of RGO sheets can be carried out according to nitrene chemistry.14 Functional groups, such as amino, bromine, and long alkyl chains, could be anchored on RGO sheets, providing a general way to produce functionalized RGO sheets. Recently, Liu et al.15 reported that the poly(diallyldimethylammonium chloride) functionalized r 2011 American Chemical Society
RGO sheets/ionic liquid composite materials could be employed as biosensors with an excellent activity for the detection of nitrate. The EDLC of GO could be enhanced by functionalization of GO with poly(sodium 4-styrensulfonate).16 Herein, functionalization of RGO with poly(diallyldimethylammonium chloride) was carried out to change the surface of RGO from a negatively charged to positively charged nature. The functionalization of RGO offered several advantages: (1) to prevent RGO from restacking after reduction due to the loss in surface oxygen-containing groups, (2) to improve the hydrophilic property of RGO, and (3) to transfer the surface from a negatively charged to positively charged nature. Manganese oxides, a class of environmentally friendly materials, are promising materials for supercapacitor applications.1720 In this work, a layered structure consisting of functinalized RGO and MnO2 nanosheets was prepared by taking advantage of the strong electrostatic interactions between them. The capacitive properties of composite material were investigated using cycle voltammetry and galvanostatic chargedischarge methods, exhibiting enhanced capacitance compared to that of pure functinalized RGO and MnO2 sheets.
’ EXPERIMENTAL SECTION Functionalization of RGO with Poly(diallyldimethylammonium chloride). In a typical preparation, GO was pre-
pared from natural graphite by using a modified Hummers Received: January 24, 2011 Revised: March 3, 2011 Published: March 21, 2011 6448
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The Journal of Physical Chemistry C method.21 GO (4 mg/mL) was diluted with 250 mL of deionized water under sonication. After 5 min, 0.4 mL of aqueous ammonia (NH3 3 H2O, 30 wt %), a given amount of poly(diallyldimethylammonium chloride) (PDDA), and 0.3 mL of aqueous hydrazine (N2H4, 35 wt %) were added to the GO suspension. Subsequently, the mixture was stirred at 95 °C for 2 h. After cooling to room temperature, the solids were separated by a centrifuge and washed with deionized water. The sample was named FRGO-p. Preparation of a Colloidal Suspension of MnO2 Nanosheets. The colloidal suspension of manganese dioxide nanosheets was synthesized according to the method described elsewhere with a slight modification.22 In a typical synthesis, 2.375 g of manganese nitrate (Mn(NO3)2) was dissolved in deionized water (40 mL). Then, 6.9 mL of H2O2 and 17.2 mL of tetramethylammonium hydroxide (TMA 3 OH) were mixed and diluted to 80 mL. The solution was added to the Mn(NO3)2 solution under vigorous stirring. The mixture was stirred at room temperature overnight. The resulting suspension was dialyzed in deionized water for 48 h. The colloidal suspension of manganese dioxide nanosheets was obtained by using a centrifuge to separate the precipitate at a speed of 8000 rpm. Dispersion of MnO2 Nanosheets on FRGO-p. MnO2 nanosheets were dispersed on FRGO-p through an electrostatic coprecipitation method. Typically, 105 mL of manganese dioxide nanosheets (0.2 mg/mL) was mixed with 105 mL of FRGO-p solution (0.085 mg/mL) under sonication. The mixture was allowed to stand at room temperature for 2 h. The resulting precipitates were washed with deionzied water and dried at 60 °C. The obtained sample was denoted as FRGO-p-MnO2. Preparation of Na-Typed Birnessite. Na-typed birnessite sheets were prepared as follows. Fifty milliliters of Na2SO4 (30 mM) solution was added to the colloidal suspension of MnO2 nanosheets (0.5 mg/mL) under sonication. Then, the mixture was allowed to stand at room temperature for 2 h. The precipitate was washed with deionic water and dried at 60 °C. The obtained sample was denoted as Na/MnO2. Characterization. Samples were characterized using powder X-ray diffraction (XRD) on a Shimadzu diffractometer (XRD6000) with Cu KR radiation at a step size of 0.02° per second. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI-5300 ESCA spectrometer (PerkinElmer) with an energy analyzer working in the pass energy mode at 35.75 eV. An Al KR line was used as the excitation source. Thermogravimetric analysis (TGA) data were collected on a thermal analysis instrument (SDT 2960, TA Instruments, New Castle, DE) with a heating rate of 10 °C/min in an air flow of 100 mL/min. The morphologies of the samples were characterized by field-emission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Japan) and high-resolution transmission electron microscopy (HRTEM, JEOL-2100F, 200 kV). The zeta-potential measurement of GO, FRGO-p, and MnO2 suspensions was carried out by means of ZetaPlus Zeta-Potential Analyzer (Brookhaven Instruments Corporation, Holtsville, NY) equipped with a 570 nm laser. The pH of the colloidal suspensions was adjusted by using sodium hydroxide or nitric acid solution. Electrochemical Measurement. Autolab PGSTAT302N was used to measure the electrochemical properties of samples at ambient temperature (about 22 °C). The electrochemical cell had a three-electrode configuration with a bright Pt plate as the counter electrode and an Ag/AgCl electrode as the reference electrode. The working electrodes were prepared by mixing active material, acetylene black, and polytetrafluoroethylene
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Figure 1. Zeta-potential profiles of GO, FRGO-p, and MnO2 nanosheets.
(PTFE) with a ratio of 85:10:5. A small amount of ethanol was added to make a homogeneous mixture. The slurry of the mixture was pressed on nickel foam and dried at 95 °C for 4 h. The mass loading of the active material for each electrode was about 2 mg. One molar Na2SO4 solution was employed as the electrolyte.
’ RESULTS AND DISCUSSION Characterization of Samples. The colloidal suspension of MnO2 nanosheets was investigated by using a UVvisible absorption spectrum. The UVvisible spectra (Figure S1, Supporting Information) showed an optical absorption with a broad peak centered at about 370 nm, attributing to the d-d transition of Mn ions in the MnO6 octahedra of MnO2 monosheets.22,23 Additionally, the concentration of MnO2 sheets is linear with the absorbance, indicating the formation of the colloidal suspension of MnO2 nanosheets. The electrostatic interaction governing the fabrication of FRGO-p and MnO2 sheets has been investigated by varying the pH of solutions. Figure 1 shows the zeta-potential profiles of surface charges of FRGO-p and MnO2 nanosheets as a function of pH. As a comparison, the zeta-potential profile of GO is shown in the same figure. In the measured pH range, GO was negatively charged due to the ionization of functional groups.24 In contrast, FRGO-p became positively charged in the studied pH range due to surface functionlization. The functionlization of RGO with PDDA would be attributed to the ππ interaction between the unsaturated CdC contaminant in PDDA chains and the basal plane of RGO,2527 transferring the surface charges from negative to positive. A similar result has been observed for the functionalized carbon nanotubes.27 The zeta-potential of the colloidal suspension of MnO2 nanosheets was investigated in the pH range of 410, revealing the negatively charged surface. Therefore, the composite of FRGO-p and MnO2 nanosheets could be prepared via the electrostatic coprecipitation method (Figure S2, Supporting Information). Figure 2 shows XRD patterns of samples. For sample GO, the sharp peak at about 2θ = 10.4° (Figure 2a) corresponds to the (002) reflection of stacked GO sheets, suggesting the introduction of oxygen-containing groups on GO sheets.28 After the chemical reduction of GO, a broad peak can be seen from sample RGO (Figure 2b). This peak was due to the residual oxygencontaining groups on the RGO sheets.29 For sample FRGO-p, no obvious peak but only humps can be observed (Figure 2c). The humps were attributed to the scattering of irregularly agglomerated FRGO-p sheets. When the amount of PDDA was increased to 0.1 wt %, a sharp peak at around 5.7° with several humps can be seen (Figure 2d), suggesting the presence of PDDA in the 6449
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Figure 2. XRD patterns of GO (a), RGO (b), and RGO functionalized with different amounts of PDDA: 0.05 wt % (c), 0.1 wt % (d), Na/MnO2 (e), and FRGO-p-MnO2 (f).
RGO interlayer after drying. Further increasing the amount of PDDA did not change the position but the sharpness of the peaks. A similar phenomenon was observed in the reassembling manganese oxide sheets with PDDA since the intercalation of the PDDA polymer into manganese oxide sheets resulted in an ordered layerstacking.30 For sample Na/MnO2, broad peaks centered at about 12.4° and 24.6° (Figure 2e) attributing to the diffraction of (001) and (002) planes of the birnessite phase (JCPDS No. 43-1456) can be seen, suggesting the formation of Na-typed birnessite.17 Sample FRGO-p-MnO2 showed an XRD pattern (Figure 2f) similar to that of Na/MnO2. However, no diffraction peaks due to FRGO-p can be seen. From the XRD data, it is believed that the assembly of the MnO2 nanosheets on the surface of the FRGO-p sheets prevented the RGO sheets from stacking. XPS was employed to characterize the surface chemical compositions and the valence states of samples. For the GO sample in Figure 3a, the main peak centered at about 284.6 eV originated from the graphitic sp2 carbon atoms. The binding energies located at 286.7 and 287.9 eV were due to carbon atoms connecting with oxygenate groups, such as CO and OCd O.31,32 The C 1s XPS spectrum of RGO (Figure 3b) exhibited that the intensity of peaks related to oxygenate groups was much lower than those of GO, suggesting the considerable deoxygenation by the reduction process. Additionally, an additional component centered at 285.6 eV was observed, which was attributed to the carbon bound to nitrogen, CN and/or CdN.29,31,33 The high resolution C 1s XPS signals of FRGO-p in Figure 3c exhibited the same functional groups as those of RGO. However, the peak centered at about 285.7 eV, corresponding to the carbon bound to nitrogen, obviously increased in intensity, indicating the higher nitrogen atomic content in FRGO-p. Core-level XPS spectra in the binding energy range of N 1s were obtained on RGO and FRGO-p. For the sample of RGO, the trace N was fitted by three component peaks centered at about 399.0 eV, 400.0 eV, and 401.9 eV in Figure 3d, corresponding to pyridinic N, pyrrolic N, and quaternary N, respectively,10 which would be
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Figure 3. High-resolution XPS spectra of C 1s for GO (a), RGO (b), and FRGO-p (c), N 1s for RGO (d) and FRGO-p (e), and Mn 2p for FRGO-p-MnO2 (f).
introduced by the chemical reduction with hydrazine. In contrast, the district profile of N peaks was observed on the spectrum of FRGO-p (Figure 3e). The dominant one centered at about 402.0 eV was mainly attributed to the quaternary N from PDDA,27 suggesting the surface functionalization of FRGO-p. The satellites would be attributed to nitrogen binding to carbon on the surface of FRGO-p. Figure 3f shows the core-level XPS signal of Mn 2p, revealing the Mn 2p3/2 and 2p1/2 centered at 642.0 and 653.9 eV, respectively, indicating that the predominant oxidation state is þ4.34,35 It should be noted that some of the Mn4þ ions at the center of the MnO6 octahedra replaced with Mn3þ ions, giving a net negative charge, led to the formation of a colloidal suspension of MnO2 nanosheets due to electrostatic repulsion,36,37 whereas the net negative charges compensated by the cations would lead to the formation of the birnessite phase (e.g., Na/MnO2).17 The morphology of FRGO-p, FRGO-p-MnO2, and Na/ MnO2 sheets was investigated using TEM and FESEM. As is seen from Figure 4a, the FRGO-p sheets are transparent as thin films. Many wrinkles and folds can be seen, showing the twodimensional structure of the RGO sheets.14 The TEM image (Figure 4b) of dried MnO2 sheets showed an aggregation of flexible flaky forms, which seemed to be a wrinkled or crumpled thin paper with many folds. Figure 4c shows the morphology of FRGO-p-MnO2, which was different from those of the pure FRGO-p and the dried MnO2 sheets. However, the profile of layered FRGO-p with a lateral dimension of micrometers was still observed in the image. Additionally, the large folds (as indicated by the arrows in Figure 4c) could be observed, attributing to the MnO2 nanosheets dispersed on the surface of FRGO-p due to the electrostatic interaction. The enlarged image in Figure 4d further confirmed the intimate contact between FRGO-p and MnO2 nanosheets with different shapes, which is important for improving electrical conductivity. The SEM image of Na/MnO2 shown in Figures 4e exhibited a porous structure with abundant surface wrinkles. The TEM image in Figure 4f revealed the layered structure of reassembled MnO2 sheets in a transparent swollen state. 6450
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Figure 4. TEM images of FRGO-p (a), MnO2 sheets (b), and FRGO-p-MnO2 (c,d). FESEM and TEM images of Na/MnO2 (e,f).
Figure 5. TGA and DrTGA curves of RGO (a), FRGO-p (b), Na/ MnO2 (c), and FRGO-p-MnO2 (d).
The thermal stability of RGO, FRGO-p, Na/MnO2, and FRGO-p-MnO2 sheets was investigated by using TGA. As shown in Figure 5a, one characteristic peak on the differential thermogravimetric (DrTG) curve was observed at about 460 °C. The regular weight loss ended at about 540 °C and was attributed to the removal of the adsorbed water, the residual oxygenate groups on the surface of RGO, and the subsequent burning of the carbon
sketch of RGO. Figure 5b shows the steps of weight loss for FRGO-p. The first step of weight loss was attributed to the removal of the interlayer water, the residual oxygenate groups, and the carbonization of PDDA chains grafted to FRGO-p sheets. The second step of weight loss at about 580 °C corresponded to the burning of the carbon sketch. The latter shifted to a higher temperature compared to RGO, indicating the improved thermal stability due to the functionalization of PDDA.14 For the sample of Na/MnO2, the weight loss in the temperature range of 50200 °C (Figure 5c) was due to the removal of absorbed water. The second weight loss at about 542 °C was attributed to the removal of the remaining interlayer water and the decomposition of MnO2 layers into Mn2O3.36 For the sample of FRGOp-MnO2, the weight loss (Figure 5d) showed the corporate features of MnO2 and FRGO-p. However, the burning of the carbon sketch was shifted to a lower temperature (around 400 °C) than that of FRGO-p, resulting from the catalytic effect of manganese oxides.18,38 Electrochemical Properties. The capacitive performances of FRGO-p, Na/MnO2, and FRGO-p-MnO2 were evaluated by cycle voltammetry (CV) and galvanostatic charge/discharge techniques in 1 M Na2SO4 solution. The measured currents were converted into gravimetric capacitances by normalizing with the electrode mass in Figure 6. Theoretically, an ideal CV curve of an electrochemical capacitor would be of standard rectangular shape since the capacitance C (C = i/mυ, i is the current, and m is the mass of active material) would keep constant at a linear charging/discharging rate (scan rate, 6451
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Figure 7. Chargedischarge curves of FRGO-p (a), Na/MnO2 (b), and FRGO-p-MnO2 (c) in Na2SO4 solution. Figure 6. Cyclic voltammograms of FRGO-p (a), Na/MnO2 (b), and FRGO-p-MnO2 (c) at different scan rates.
υ = dE/dt = constant, E is the potential window of charge/ discharge, and t is the time of charge/discharge).19 Figure 6a shows the CV curves of FRGO-p at the different scan rates. At a low scan rate, FRGO-p exhibited a quasi-rectangle shape with a pair of broad redox peaks appearing at about 0.1 and 0.05 V, resulting from the reversible oxidation and reduction of functional groups on the surface of FRGO-p sheets.3942 CV curves of Na/MnO2 exhibited the slightly redox peaks, suggesting the pseudocapacitance of Na/MnO2.19 Additionally, the current was delayed to reach a horizontal value after the reversal of the potential sweep at a higher scan rate, indicating the increasing resistance. CV curves of FRGO-p-MnO2 shown in Figure 6c exhibited rectangle shapes with small redox peaks, the typical behaviors of the combination of EDLC of RGO and pseudocapacitance from the redox reaction of MnO2. On the basis of the CV curves, we can see that the FRGO-p-MnO2 exhibited the highest capacitance at a constant scan rate than those of pure FRGO-p and Na/MnO2, indicating the synergistic effect of FRGO-p and MnO2 nanosheets. The galvanostatic charge/discharge curves of FRGO-p, Na/ MnO2, and FRGO-p-MnO2 in 1 M Na2SO4 were shown in Figure 7. All the curves exhibited an equilateral triangle shape, indicating the good reversibility during the charge/discharge process. However, at the same current density, the Na/MnO2 electrode exhibited a more pronounced potential drop (IR drop) at the beginning of the discharge process. The potential drop is attributed to the internal resistance of the electrode associated with the electrical connection resistance, bulk solution resistance, and resistance of ion migration in electrode materials.3 As an example, the charge/discharge curves at the current density of
Table 1. Specific Capacitances of Electrodes Na/MnO2, FRGO-p, and FRGO-p-MnO2 Calculated from the Charge/ Discharge Curves Measured at Different Current Densities specific capacitances (F/g) samples
4 A/g
2 A/g
1 A/g
0.5 A/g
0.25 A/g
FRGO-p
49
56
66
70
74
Na/MnO2 FRGO-p-MnO2
46 130
71 142
95 168
119 179
140 188
1 A/g shown in Figures S3aand b (Supporting Information) revealed the IR drop of 0.1 V for Na/MnO2 and 0.04 V for FRGO-p-MnO2, respectively. As shown in Figure S3c (Supporting Information), the IR drop increased linearly with increasing current density. The internal resistance of electrodes could be roughly estimated from the slope of this linear relationship.43 The calculated resistance of FRGO-p-MnO2 (about 0.04 Ω g) was over two times smaller than that of Na/MnO2 (about 0.09 Ω g), indicating the great decrease in the resistance of FRGO-p-MnO2. On the basis of the charge/discharge curve, the specific capacitance can be calculated by using the equation:5,34 C = iΔt/mΔE, where i in A is the discharge current, Δt in s is the discharge time, m in g is the mass of the active electrode material, and ΔE in volts is the voltage range of discharge after IR drop. The specific capacitances of FRGO, Na/MnO2, and FRGO-pMnO2 are summarized in Table 1. A specific capacitance of 188 F/g for FGRO-p-MnO2 was achieved at the current density of 0.25 A/g, higher than 74 F/g for FRGO-p, 140 F/g for Na/ MnO2. Recent studies10,11 have shown that deposition of metal oxides, such as RuO2 nanoparticles and Ni(OH)2 nanoplates, on 6452
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The Journal of Physical Chemistry C RGO sheets can enhance the capacitive performance. It has also been reported that thin films consisting of polyaniline fibers and RGO sheets exhibited an improved capacitive performance.44 These improvements are due to contributions of the high electrical conductivity of RGO and the pseudocapacitance of the metal oxides or conducting polymer. In the present work, the remarkable enhancement in the specific capacitance of sample FGRO-p-MnO2 can be similarly attributed to the contributions of both components. Anchoring of the MnO2 nanosheets on the FRGO-p sheets effectively prevent the latter from agglomeration, thus facilitating ion transport in the electrode material, and eventually improving the electric double-layer capacitance. The effective charge transfer in the electrode played an import role in enhancing the specific capacitance and rate capability. For sample FRGO-p-MnO2, the face-to-face assembly between the MnO2 nanosheets and the FRGO-p sheets with a good electrical conductivity results in an intimate interaction of both components, enhancing charge transfer between the two components and leading to rapid redox reactions of the MnO2 sheets. The rate performance of the samples was evaluated by comparing the specific capacitances, and the data are shown in Table 1. It is interesting to note that sample Na/MnO2 performed similarly to that of FRGO-p at high current density. However, sample Na/MnO2 performed much better with decreasing current density. At the current density of 0.25 A/g, the specific capacitance of Na/MnO2 (140 F/g) was about two times that of FRGO-p (74 F/g). Considering the different electrode materials, the difference in the specific capacitances of two electrode materials was due to the different charge storage mechanisms. The charge storage in Na/MnO2 was mainly based on the faradic reaction of manganese oxide according to a pseudocapacitive mechanism.6 At the low current density, low concentration polarization allows the charging process to reach completion after a long time. Ions in the vicinity of the electrodeelectrolyte interface would be enough, thus improving the redox transitions of manganese oxide, eventually leading to a high capacitance.5 For the electrode of FRGO-p, this insignificant dependence of specific capacitance upon the current density was due to the fact that EDLC was the main contribution to the observed capacitance. Therefore, the Na/MnO2 electrode in the Na2SO4 electrolyte showed a better capacitive performance at the lower current density. Importantly, the situation could be greatly enhanced by the combination of EDLC from FRGO-p and pseudocapacitance from MnO2 nanosheets. Indeed, the enhanced capacitances were obtained by the incorporation of FRGO-p and MnO2 nanosheets. The specific capacitances of FRGO-p-MnO2 (130 F/g) were about three times those of Na/ MnO2 (46 F/g) at the current density of 4.0 A/g, although the capacitance of Na/MnO2 (140 F/g) is comparable with that of FRGO-p-MnO2 (188 F/g) at the lower current density of 0.25 A/g. The higher capacitance retention ratio of over 69% for FRGO-p-MnO2 was obtained compared to that of Na/MnO2 (only 33%), when the current density was increased by 16 times (from 0.25 A/g to 4.0 A/g). The enhanced capacitive performances at a high rate would result from the low resistance because the synergic effect of the FRGO-p and MnO2 sheets affords facile ion and charge transfer in the electrode materials, which was evident by the investigation of EIS. As shown in Figure 8, Nyquist plots of FRGO-p, Na/MnO2, and FRGO-pMnO2 exhibited a semicircle over the high frequency range, followed by a linear part in the low frequency region. It should be noted that the large semicircle observed for the electrode is
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Figure 8. Electrochemical impedance spectra (EIS) of Na/MnO2, FRGO-p, and FRGO-p-MnO2. The inset shows the enlarged EIS at the low frequency region.
indicative of high interfacial charge-transfer resistance, attributing to the poor electrical conductivity of materials, whereas the more vertical line corresponds to an electrode more close to an ideal capacitor.43 The equivalent series resistance (ESR) of FRGO-p, Na/MnO2, and FRGO-p-MnO2 obtained from the intersection of the Nyquist plot at the x-axis is 5.5 Ω, 14.0 Ω, and 5.8 Ω, respectively. Considering the similar morphology of FRGO-p, Na/MnO2, and FRGO-p-MnO2 under the same experimental conditions, we would attribute the difference in the ESR of electrodes to the different conductivities of electrode materials. The low conductivity of Na/MnO2 resulted in the significant charge transfer resistance among Na/MnO2. In contrast, FRGO exhibited a smaller ESR due to the good conductivity. The ESR of FRGO-p-MnO2 was smaller than that of Na/ MnO2, comparable with that of FRGO-p, suggesting the decreased charge transfer resistance in the presence of FRGO-p. The Nyquist plot at high frequency (the straight line) is the resistance resulting from the frequency dependence of ion diffusion/transport. The increase in the impedance of this region indicates greater variations in ion diffusion path lengths and increased obstruction of ion movement.45 Therefore, the high resistance of ion transfer in pure FRGO-p would be attributed to the high charge density (as indicated by the zeta-potential measurement), resulting in the low capacitance. In contrast, FRGO-p-MnO2 exhibited a short diffusion path length of ions in the electrolyte, which could be seen from the low resistance of the capacitive part on the Nyquist plot. This may be illuminated by the positive effect of the composite: the formation of the FRGO-p-MnO2 composite resulted in the surface charge FRGOp being compensated by the negative charges from MnO2 nanosheets, leading to a lower resistance of ion transfer. Moreover, the presence of FRGO-p with high electrical conductivity resulted in a lower resistance of charge transfer. The advantages of composite electrode FRGO-p-MnO2 over the pure FRGO-p and MnO2 were clearly demonstrated. Additionally, the cycling stability of the FRGO-p-MnO2 electrode in Na2SO4 solution between 0 to 0.9 V was investigated at a current density of 4.0 A/g. As shown in Figure 9, the capacitive retention was about 89% after 1000 cycles, indicating a good cycling ability of the composite materials. The support carbon matrix FRGO-p allowed the deposition of the MnO2 nanosheets on the surfaces of FRGO-p sheets, which would enhance the mechanical strength of composite materials, resulting in the long charge/ discharge cycling ability. 6453
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Figure 9. Capacitance retention of FRGO-p-MnO2 at a current density of 4.0 A/g in 1 M Na2SO4 electrolyte. The inset shows the charge/ discharge curves of FRGO-p-MnO2 at a current density of 4.0 A/g.
’ CONCLUSIONS In summary, functionalized RGO (FRGO-p) was prepared by the reduction of functionalized graphene oxide with poly(diallyldimethylammonium chloride) (PDDA), transferring the surface charge of RGO from negative to positive. A composite of FRGO-p and MnO2 nanosheets (FRGO-p-MnO2) was synthesized by anchoring negatively charged manganese dioxide MnO2 nanosheets on the positively charged FRGO-p via an electrostatic coprecipitation method. Importantly, FRGO-p-MnO2 exhibited enhanced capacitive performances than those of pure FRGO-p and Na/MnO2 sheets, attributing to the contributions of the good electrical conductivity of FRGO-p and the pesudocapacitance of the MnO2 nanosheets. Additionally, over 89% of the original capacitance was retained after 1000 cycles, indicating a good cycle stability of composite materials. ’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed description of the synthesis of graphene oxide, UVvisible spectra, and the calculation of resistance of electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: þ65 65164727. Fax: þ65 67791936. E-mail: chezxs@ nus.edu.sg.
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dx.doi.org/10.1021/jp200724h |J. Phys. Chem. C 2011, 115, 6448–6454
