Improve the Supercapacity Performance of MnO2-Decorated

Article pubs.acs.org/JPCC

Improve the Supercapacity Performance of MnO2‑Decorated Graphene by Controlling the Oxidization Extent of Graphene Ying Li, Naiqin Zhao, Chunsheng Shi,* Enzuo Liu, and Chunnian He School of Materials Science and Engineering, and Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin300072, China ABSTRACT: Metal oxide decorated graphene nanocomposites have been explored as potential electrode materials for supercapacitors. In the present work, the effects of the oxidation degree of the graphene on the loading of needle-like nano-MnO2 and electrochemical performance of the MnO2− graphene composites were systemically investigated. MnO2− graphene composites were prepared for supercapacitor electrodes via hydrazine reduction of graphite oxide and a soft chemical precipitation route at relatively low temperature. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) characterizations confirm the decrease of oxygen-containing functional groups with the increase of reducing time. XRD, SEM, and TEM studies show the nanostructure and micromorphology of the prepared composites and indicate the crystallographic structures of α-MnO2. Cyclic voltammetry (CV) and galvanostatic charge−discharge measurement were used to evaluate electrochemical properties of the composites, and the specific capacitance values were in the range of 74.8−124 F/g at 200 mA/g with 1 M Na2SO4 as the electrolyte. The oxygen-containing functional groups may increase the resistance of the composites, but benefits the dispersion of MnO2 on graphene and the infiltration of electrolyte. Therefore, controlling the oxygen content of graphene plays an important role in the fabrication of high energy-density graphene-based supercapacitor materials.

1. INTRODUCTION Energy storage/conversion devices have been intensely researched in recent years due to the increasingly serious environmental issues and dwindling of fossil resources.1−3 Electrochemical capacitors or supercapacitors (SCs) combine the advantages of electrolytic capacitors, which can deliver a high power in a very short time, with batteries, which store high energy, and fill the vacancy between batteries and conventional capacitors.4−8 SCs have a long cycle life and a rapid charging and discharging rate at high power densities9−11 because their mechanism of energy storage is the simple charge-separation or fast and reversible redox reactions at the interface between the electrode and the electrolyte.12−16 Tremendous attention has been paid to SCs owing to their wide and growing range of potential applications, such as short-term power sources for mobile devices, hybrid electric vehicles, etc.3,16 There are three main categories of electrode materials: carbon based, metal oxides, and polymeric materials.2,17−19 A series of Faradaic redox reactions occur in metal oxides and polymeric materials during charge and discharge in pseudocapacitors.8,20 However, charge storage on carbon electrodes is predominantly capacitive in the electrochemical double layer, namely, the simple charge-separation.8,21 Carbon as a material for the storage of energy in supercapacitors is highly attractive because of its accessibility, high surface area, good processability, and relatively low cost.20−22 Different species of carbonaceous materials have been used as electrode materials © 2012 American Chemical Society

such as amorphous carbons, activated carbon, and carbon nanotubes.23,24 In recent years, graphene, a novel unique carbon material with one-atom thick layer 2D structure, has emerged as a new class of promising materials in supercapacitors.15,25−27 In addition, for the purpose of further improving the interfacial capacitance, hybrid graphene-based composites have also been investigated by combining pseudocapacitive materials, such as polyaniline or metal oxide.11,28,29 Hydrated RuO2 has been considered to be one of the most promising pseudocapacitor materials among all the transition metal oxides.16 However, the high cost of RuO2 has stimulated the researchers to focus on other transition metal oxides such as MnO2, NiO, etc.3,30 Because of its environmental compatibility, low cost, and abundant availability, MnO2 is regarded as a promising electrode material for applications in SCs and has attracted extensive attention.25,31,32 It is known that in such a hybrid material with pseudocapacitive materials decorated on graphene nanosheets, the graphene nanosheets play two roles: ensuring fast electron transport through the SC electrode and acting as an ideal substrate for growing and anchoring functional nanomaterials.33−35 Graphite oxide as graphene’s derivative is an ideal matrix for growth of functional nanomaterials.34,35 Even though the Received: August 2, 2012 Revised: November 2, 2012 Published: November 14, 2012 25226

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Figure 1. XPS spectra of C1s for (a) GO, (b) 1hRGO, (c) 4hRGO, and (d) 12hRGO.

room temperature for 2 h. Then 0.1 mL of hydrazine monohydrate was added to the suspension, and the mixture was refluxed at 80 °C for different times from 15 min to 24 h. Once the reaction was completed, the RGO was collected as black powder after the mixture was filtrated and washed with distilled water. The final product was transferred to 100 mL of distilled water and ultrasonicated for 2 h. The achieved suspension was dried by freeze-drying and denoted as 15minRGO, 24hRGO, and so on. 2.3. Synthesis of MnO2/Graphene Composites. Nanocomposites comprising MnO2 and RGO with different degrees of oxidation were synthesized according to the reported method.35 XRGO (0.066 g) (X = 15 min, 1 h, 2 h, 4 h, and 8 h) and MnCl2·4H2O (0.27 g) were dispersed in isopropyl alcohol (50 mL) with ultrasonication for 2 h. Then the suspension was transferred to a water bath at 83 °C with magnetic stirring, and 5 mL of distilled water with dissolved KMnO4 (0.15 g) was poured into the mixture. After reaction under stirring and refluxing for 30 min, the product was vacuum filtrated and washed with distilled water several times to remove the isopropyl alcohol. Finally the powder was obtained after drying at 80 °C in a vacuum oven, grinding and marked as XRGM (X = 15 min, 1 h, 2 h, 4 h and 8 h). 2.4. Characterization. The microscopic feature and morphology of the samples were observed by field emission scanning electron microscope (FESEM, HITACHI S4800) operated at 10 kV and high-resolution transmission electron microscope (HRTEM, PHILPS TECNAI G2 F20). The crystallographic structures of the products were determined by a powder X-ray diffraction system (XRD, Rigaku D/max2500, Cu−Kα radiation) at room temperature. The X-ray photoelectron spectroscopy (XPS) characterization was carried out on a PHI 1600 spectrometer equipped with a monochromatic Al Kα X-ray source. The electrical conductivity of the graphene with different oxidation degrees was measured by a four-probe method. Thermogravimetry (TG) analyses

decoration of nanoparticles on graphite oxide (GO) or graphene sheets and its application in SCs have been investigated,9,18,35 little attention has been paid to study the electrochemical properties of nanoparticle decorated graphene sheets with different degrees of oxidation. Herein, we have systematically investigated the decoration of MnO2 nanoparticles on graphene with different degrees of oxidation and the electrochemical properties of the composites. It is found that the electrochemical properties of the composites are seriously affected by the degree of oxidation of the graphene substrates.

2. EXPERIMENTAL SECTION 2.1. Preparation of Graphite Oxide. GO was synthesized from natural graphite by a modified Hummers method.36 Graphite (3 g) was mixed with concentrated H2SO4 (98%, 120 mL) in a 1000 mL beaker within an ice bath. KMnO4 (15 g) was added gradually with continuous magnetic stirring and cooling. The rate of addition was carefully controlled to keep changes of the reaction temperature within 4 °C. After stirring for 2 h, the mixture was moved to a 35 °C water bath and stirred for 1 h. Then the temperature was increased to 60 °C and distilled water (250 mL) was added slowly with vigorous stirring to hold the temperature at 60 ± 3 °C. After reaction for 30 min, distilled water (500 mL) and 30% H2O2 solution (30 mL) were added with constant stirring. The suspension was centrifuged and washed with 10% HCl aqueous solution until sulfate could not be detected with BaCl2. The product was dried under vacuum at 80 °C for 24 h, and the GO was obtained as a gray powder after grinding. 2.2. Preparation of Reduced Graphene Oxide with Different Degrees of Oxidation. Chemical conversion of GO to reduced graphene oxide (RGO) with different degrees of oxidation was accomplished according to the reported method.37,38 In a typical experiment, 0.1 g of graphite oxide was dispersed in 100 mL of distilled water by ultrasonication at 25227

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were carried out on NETZSCH STA 449C from 40 to 900 °C with a heating rate of 10 °C/min in air flow to detect the actual mass ratios of MnO2 to RGO in composites. 2.5. Electrochemical Evaluation. The galvanostatic charge−discharge measurement was performed on a Land Battery workstation (Wuhan Land Instrument Company, China) at ambient temperature. And the cyclic voltammogram (CV) was measured on a CHI 660D electrochemical workstation (Shanghai CH Instrument Company, China). The electrochemical characterizations were carried out in a three-electrode cell with a platinum plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode in 1 M Na2SO4 electrolyte at room temperature. The working electrode was prepared from a homogeneous paste, containing 75 wt % the prepared material, 15 wt % acetylene black, and 10 wt % polytetrafluoroethylene (PTFE) binder dispersed in a few drops of distilled water. The mixture was coated on nickel foam and vacuum-dried at 80 °C overnight. The mass of the active material was calculated by the increase in weight before and after coating. The working electrodes were tested after immersion in 1 M Na2SO4 solution for about 12 h.

Figure 3. XRD patterns of (a) GO, (b) 1hRGO, (c) 8hRGO, and (d) 8hRGM.

3. RESULTS AND DISCUSSION 3.1. Investigation of the Oxidation Degree of Graphene. Figure 1 shows the C1s XPS spectra of GO Table 1. Percentage of the Peak Area at Each Binding Energy (eV) in the XPS Spectra of C1s binding energy

GO

1hRGO

4hRGO

12hRGO

284.6 286.6 288 289.5

52.47 37.70 9.38 0.45

53.99 36.34 7.96 1.71

67.68 23.47 6.54 2.31

66.89 22.96 7.03 3.12

Figure 4. FESEM images of (a) GO, (b) 8hRGO, (c) pure MnO2, and (d) 4hRGM. Inset in panel d shows the MnO2 nanoneedles grown between the graphene layers.

(Figure 1a), 1hRGO (Figure 1b), 4hRGO (Figure 1c), and 12hRGO (Figure 1d). The spectra can be divided into four components according to carbon valence in different oxygencontaining functional groups: nonoxygenated C at 284.6 eV originated from the graphitic sp2 carbon atoms; the binding energy of carbon in C−O located at 286.6 eV; the component centered at 288 eV attributed to carbonyl carbon (CO), and the peak observed at 289.5 eV obtained on the carbon bound in carboxyl (O−CO).9 The percentage of each binding energy listed in Table 1 suggests the different degrees of deoxygenation of graphene oxide. Accordingly, the extent of oxidation of GO decreases with increasing the reduction time. Additionally, in this work, a FTIR technique was used to confirm the shift of the oxygen groups. Figure 2 shows the vacuum FTIR transmittance spectra (KBr) of GO and samples with different reduction time. The FTIR spectrum of GO has peaks around 3425, 1718, and 1615 cm−1, corresponding to O−H stretching, CO stretching vibrations from carbonyl and carboxylic groups, and CC stretching vibrations from unoxidized graphitic domains, respectively, and the bands at 1300−1000 cm−1, corresponding to C−O stretching vibrations.30,33 It is obvious that the intensity of the peaks around

Figure 2. FTIR spectra of (a) GO, (b) 30 minRGO, 1hRGO, 12hRGO and 24hRGO.

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Figure 6. CVs of (a) 1hRGM, 4hRGM, 8hRGM, and 16hRGM at 10 mV/s and (b) 8hRGM at 5, 10, 20, and 50 mV/s in 1 M Na2SO4 solution.

Figure 5. TEM images of (a) GO, (b) 8hRGO, (c) 1hRGM, (d) 4hRGM, (e) 8hRGM, and (f) 16hRGM. Insets in panels b and c show the HRTEM images of 8hRGO and MnO2 nanoneedles decorated on the RGO sheets, respectively.

to the reduction, 8hRG (Figure 4b) has more wrinkles and folds of edges than GO (Figure 4a). As shown in Figure 4c, needle-like MnO2 is decorated on the GO and is seriously aggregated. However, in 8hRGM (Figure 4d), MnO2 nanoneedles are homogenously dispersed on the surfaces or between the graphene layers. TEM images of GO and 8hRG reveal that graphite oxide layers are flat, but the sheets of 8hRG are full of folds with thickness of 4−6 atomic layers (inset in Figure 5b), as shown in Figure 5a,b. In Figure 5e, it can be seen that the MnO2 nanoneedles uniformly dispersed on the surfaces of 8hRGM. The results are consistent with SEM observation. Thus, the oxygen-containing functionalities (epoxide, hydroxyl, carbonyl, and carboxyl groups) on the graphene sheets are vulnerable to oxidation, resulting in easy MnO2 precipitation near the defect sites.39 3.4. Electrochemical Properties. The capacitive performances of RGM with different reduction time were investigated by cyclic voltammetry (CV) and galvanostatic charge− discharge techniques in 1 M Na2SO4 solution. In Figure 6a, it could be observed that all of the CV curves have similar and nearly rectangular shapes without any peaks, suggesting an excellent capacitive behavior and a low contact resistance in the supercapacitor electrode.4,40 It is obvious that the area of 8hRGM is larger than that of the others, and the area of 1hRGM is the least. Figure 6b shows the CV curves of 8hRGM collected at various scan rates from 5 up to 50 mV/s, and the linear current with the increase of voltage indicates that there is little faradic in the supercapacitor electrode.15 The CV curves

3425 and 1718 cm−1 are gradually weakened with the increase of the reducing time, and at the same time, the bandwidth at 1300−1000 cm−1 is wider and the intensity of the peak around 1615 cm−1 increases. The results indicate that most of the carboxylic groups and hydroxyl functional groups on the GO nanosheets are gradually removed with the increase of the reduction time, and as a result, unoxidized graphitic domains and the content of C−O increase, which was consistent with the XPS results. 3.2. XRD Analysis. Figure 3 displays the XRD patterns of GO, 1hRGO, 8hRGO, and 8hRGM. The sharp peak in Figure 3a at about 2θ = 10.24° corresponds to the (001) reflection of stacked GO sheets, and the interlayer spacing (0.87 nm) is much larger than that of pristine graphite (0.34 nm), suggesting the presence of oxygen-containing groups on GO sheets. After the chemical reduction of GO, a broad (002) peak at about 2θ = 23° can be detected from the sample 1hRGO or 8hRGO (Figure 3b,c), which was due to the removal of the oxygen intercalated into the interlayer spacing of graphite and restoration of a small number of graphitic structures. After decoration by MnO2, however, the (002) reflection peak of graphite has almost disappeared as shown in Figure 3d suggesting that the surfaces of RGO are fully covered by MnO2 and the diffraction peaks of the obtained composites (8hRGM) indicate a tetragonal phase of α-MnO2. 3.3. Morphological Studies. The SEM images of GO, 8hRG, pure MnO2, and 8hRGM are shown in Figure 4. Owing 25229

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Figure 8. TG curves of MnO2/graphene nanocomposites under air atmosphere.

CS = I Δt /(ΔVm)

(F/g)

(1)

In this formula, I represents the constant discharge current, Δt is the discharge time, ΔV is the potential drop during discharge, and m is the quality of the active materials in each electrode. These results shown in Figure 7b are mainly consistent with the order specified by the CVs in Figure 6. The maximum energy density (Emax) and the specific power density (P) of the electrodes were calculated by using the following eqs 232 and 3:30 Emax =

P=

1 C V2 2 s

I ΔV m

3.6

(W·h/kg)

(W/kg)

(2) (3)

where V is the operating potential window (1.0 V), I represents the constant charge−discharge current, ΔV is the potential drop during discharge, and m is the quality of the active materials in each electrode. The energy density of pure MnO2, 1hRGM, 2hRGM, 4hRGM, 8hRGM, and 16hRGM were calculated to be 10.4, 12.2, 14.4, 16.0, 17.2, and 16.5 W·h/kg, respectively, at a power density of 200 W/kg. The long-term cycle stability of the 8hRGM composite electrode was also examined by galvanostatic charge/discharge measurements at 200 mA/g for 1000 cycles in 1 M Na2SO4 solution (Figure 7c). It is found that the specific capacitance values decreased with respect to the number of cycles because of the loss of the active material and the capacitance retention rate was 78.7% of the initial after 1000 cycles. The specific capacitances and energy density of 8hRGM are higher than the reported values for CNT-based supercapacitor devices,30,32 although the long-term cycle stability is somewhat worse than previously reported.32,35 As well known, the charge storage mechanism of MnO2based electrodes in mild electrolytes is due to the rapid intercalation of alkali metal cations in the electrode during charge and discharge upon oxidation and reduction: MnO2 + C+ + e− = MnOOC, where C = Na, K, and Li. It should be noticed that the proposed mechanism involves a redox reaction between the III and IV oxidation states of Mn.31 Increasing the contact area of the active materials and the electrolyte may contribute to the improvement of the capacitive performance.15 To investigate the impact of reduction time on the formation of manganese oxide, we compared the actual mass ratios of MnO2 to RGO in composites by TG measurements. Figure 8 displays

Figure 7. (a) Comparison of galvanostatic curves of pure MnO2, 1hRGM, 2hRGM, 4hRGM, 8hRGM, and 16hRGM at 200 mA/g; (b) The value of specific capacitance calculated at 200 and 500 mA/g; (c) long-term cycle stability of the 8hRGM in 1 M Na2SO4 at 200 mA/g.

are close to rectangular at various scan rates, indicating an excellent capacitance behavior and low contact resistance in the capacitors.3,15 Because of the internal resistance of the composites electrode, the curve shape is gradually distorted from rectangular with the increase of the scan rate.15,18 To obtain more information about the nanocomposites as electrode materials for supercapacitors, galvanostatic charge/ discharge measurements were carried out in 1 M Na2SO4 between 0 and 1 V at a current density of 200 mA/g. Figure 7a shows the charge and discharge curves of the synthesized composites at 200 mA/g. The linear and symmetrical curves in the total range of potential indicate a very good capacitive behavior corresponding to the CV curves. The specific capacitances (Cs) of the electrodes were calculated from the discharge curves using eq 1,9,15,35 and plotted in Figure 7b. 25230

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Table 2. Electronic Conductivity of the Graphene with Various Oxidation Degrees samples

GO

1hRGO

2hRGO

8hRGO

12hRGO

24hRGO

σ (S/cm)

3.208 × 10−4

9.304 × 10−2

1.009

1.559

1.315

0.761

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the TG curves of the composites and the mass loss values (in brackets), which indicate that manganese dioxide loading decreases gradually with the reduction time increasing. The electronic conductivity values of the graphene with different oxidation degrees are shown in Table 2. The electronic conductivity of the GO is several orders of magnitude lower than the others’ and the conductivity increases rapidly with increasing the reduction time till 8 h. That 8hRGM is more favorable for supercapacitors may be due to two facts: on the one hand, the removal of oxygen functional groups on the surface may affect the electron transfer involved in the oxidation and reduction reaction of electroactive compounds. On the other hand, the remaining oxygen functional groups are helpful to the dispersion of MnO2 nanoparticles on graphene and the infiltration of electrolyte. In this viewpoint, the longer reducing time is not conducive to the capacitive performances. Therefore, suitable reducing time is needed to optimize the performance of the supercapacitor using MnO2−graphene as the electrode.

4. CONCLUSIONS In summary, we have successfully prepared the MnO2/ graphene nanocomposites with different oxidation degrees and systemically investigated the effects of graphene oxidization on supercapacity performance. It is found that 8hRGM has the best capacitive performance with the specific capacitance of 124 F/g at 200 mA/g and the energy density of 17.2 W·h/kg at a power density of 200 W/kg, which may be attributed to the pros and cons of oxygen-containing functional groups on the graphene sheets. As surface defects on graphene sheets, the oxygen-containing functional groups may increase the resistance, but they are also conducive to providing anchoring sites for MnO2 nanoneedles and better utilization of the electroactive surface for Faradaic reactions. Therefore, controlling the oxygen content on the graphene sheets plays an important role in obtaining a high energy density supercapacitor material.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 22 27891371. Fax: +86 22 27891371. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support by the National Basic Research Program of China (No. 2010CB934700), National Natural Science Foundation of China (No. 51071107), and the Innovation Foundation of Tianjin University.

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REFERENCES

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