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The Tuning of Phase, Morphology and Performance of Graphene Oxide/Manganese Oxide for Supercapacitors

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The Tuning of Phase, Morphology and Performance

of Graphene Oxide

/

Manganese Oxide for Supercapacitors

Fangfang Ding

+1

, Na Zhang

+2

, Cheng Zhang

+2

and Changwei Zhang

+1 Shanghai Institute of Technology, Shanghai 201418, P. R. China

The reduced graphene oxide (rGO)/manganese oxides (MnO2) composites are obtained from GO/MnO2precursor by the

microwave-assisted reducing procedures. Characterization indicates that the phase and morphology transformation have been discovered in GO/MnO2

composite for thefirst time, and this transition can be explained by a“dissolution-precipitation”model. The capacitive properties of the rGO/ MnO2electrodes are measured using cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) tests. The rGO/Mn3O4in hydrazine

hydrate (H-rGO/Mn3O4) exhibits a specific capacitance as high as 324.9 F/g, which is about four times as the GO/MnO2precursor (81.25 F/g).

It is deduced that the diversity of ionic state of Mn3O4, the large specific surface area, and the large reduction degree of GO are favorable to the

enhancement of the electrochemical performance. The results show that the H-rGO/Mn3O4nanocomposite from GO/MnO2can be used as

electrode material for high performance supercapacitors. [doi:10.2320/matertrans.M2015101]

(Received March 10, 2015; Accepted August 10, 2015; Published October 25, 2015)

Keywords: graphene, manganese dioxide, manganese tetraoxide, supercapacitors, electrical performance

1. Introduction

In the 21st century, due to the deteriorative environment and the depletion of fossil fuels,1­3) the new energy storage devices and energy materials with high power and energy densities have become one of the most important tasks. As an intermediate system between dielectric capacitors and batteries, supercapacitors have attracted much attention because of their high specific energy density, long cycle stability and environmental friendliness.4­8)

Graphene is a newly reported carbon material that has received tremendous attention because of its unique structure, amazing electrical and mechanical properties, since Geim succeeded in isolating it from graphite.9­12) It has applica-tions11,12) in wide areas including catalysts, solar cell, gas sensor, fuel cell, battery, supercapacitor, antibacterial study, and soon. On the other hand, as one of the green supercapacitor electrode materials, MnO2 shows greatly potential to replace RuO2due to its high specific capacitance, environmental compatibility, low cost, and abundance in nature.13,14) The introduction of MnO2 into the interlayer of graphene can prevent the agglomeration of grapheme,15) and the graphene/MnO2composite is expected to show the good electrochemical performance.16)Thus, GO supported on manganese oxide in electroactive materials for supercapaci-tors has been deeply investigated.

In the GO/MnO2 composites, lots of attempts have been made to tailor MnO2 morphologies, including nanneedle,10) nanorod17) nanosheet,18) to give the higher specic surface area to enhance the specific capacitance. On the contrast, to improve the conductivity of GO/MnO2composites, GO with different oxidation levels have been fabricated using different reducing agents.19­21) From the above results, it is deduced that the reducing degree of GO and the texture of MnO2play important factors in the improvement of specific capacitance. However, the influence of the reducing process with the help of the microwave radiation on the phase, morphology and the

electrochemical property in rGO/MnO2 samples is rarely reported.22)

In this paper, the phase transformation and morphology change have occurred in the GO/MnO2 precursor by the microwave-assisted reducing process. A series of measure-ments including X-ray diffraction (XRD), Raman spectrosco-py, Thermal Gravimetric Analyzer (TGA), Scanning electron microscopy (SEM) and Brunner-Emmet-Teller (BET) are employed to detect the change in the crystalline and morphology of the composites. The influence of different reducing process, (including hydrazine hydrate, sodium borohydride, vitamin c (Vc) and hydrothermal reaction) on the electrochemical performance have been investigated in details.

2. Experimental

2.1 Synthesis of GO/MnO2precursor

All the reagents were analytical grade and used without further purification. GO was synthesized from graphiteflakes by a modified Hummers method as described previously.23) In the typical synthesis procedure of the GO/MnO2precursor with feeding ratio of 15%, GO and MnCl2·4H2O were dispersed in isopropyl alcohol with ultrasonication for 0.5 h. Subsequently, the mixture was heated to approximately 83°C in a water-cooled condenser with vigorous stirring. Then, KMnO4 was added and kept stirring and refluxing for 0.5 h. In the end, the obtained product was filtered, washed and dried in a vacuum oven at 60°C for 24 h.

2.2 Synthesis of the rGO/MnO2composites

20 mmol hydrazine hydrate was dropped into the solution (50 ml) containing 0.05 g of GO/MnO2 precursor, and then kept in microwave synthesis system at 300 W for 5 min. The products reduced by hydrazine hydrate (namely H-rGO/ Mn3O4) was filtered, washed and dried in vacuum oven at 60°C for 24 h. The target products (abbreviated S-rGO/ Mn3O4 and V-rGO/MnO2, respectively) reduced by other reducing agents including sodium borohydride (20 mmol), and Vc (20 mmol), were obtained by the similar procedure. It +1Graduate Student, Shanghai Institute of Technology

+2Corresponding author, E-mail: nzhang@sit.edu.cn, czhang@sit.edu.cn

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is noted that the sample by hydrothermal reaction (T-rGO/ MnO2) was treated at 150°C for 5 min in a 100 ml Tefl on-lined stainless steel autoclave.

2.3 Characterization

The crystallographic structure and vibrational properties of the composites were determined by XRD (Shimadzu X-ray diffractometer 6000, Cu Ka radiation) with a scan rate of 5°/min. Raman spectra were measured on a Laser Raman spectroscopy (HORIBA JobinYvon LabRAM HR800) with 532 nm line of an Ar ion laser as an excitation source. Morphologies of as-obtained materials were observed on a field emission SEM (FESEM, Hitachi S4800) at an accelerating voltage of 20 kV. TGA was performed up to a temperature of 950°C under air flow at a heating rate of 10°C/min using a TGA-2050 instrument. BET surface area measurements were recorded with the Quantachrome Quad-rasorb automatic volumetric instrument.

2.4 Electrochemical measurements

The electrochemical property and capacitance measure-ments of the supercapacitor electrodes were characterized with cyclic voltammetry (CV) and charge-discharge (GCD) experiments using CHI660D workstation at 25°C. The working electrode was fabricated by mixing the active materials, acetylene black, and polytetrafluoroethylene (PTFE) with a mass ratio of 8 : 1 : 1. A small quantity of ethanol was added slowly into the mixture to form slurry. The resulting mixture was coated onto a nickel foam substrate and dried in an oven. It was pointed out that the rGO(3 mg), MnO2(3 mg), GO/MnO2(6 mg), T-rGO/MnO2(4 mg), V-rGO/MnO2(3 mg), S-rGO/Mn3O4(3 mg) or H-rGO/ Mn3O4(3 mg) were used in the electrochemical test. The electrochemical tests were conducted with a conventional three electrode system in 0.5 M Na2SO4 electrolyte. Typi-cally, the composite electrode was used as the working electrode, a platinum wire as the auxiliary electrode, and a saturated silver chloride electrode as the reference electrode.

3. Results and Discussion

3.1 Structural analysis

Figure 1 shows the XRD patterns of GO based compo-sites. As shown in Fig. 1(a), the most intensive peak at around 2ª=10.24° corresponds to the (001) reflection of GO. The interlayer space (0.87 nm), much larger than that of pristine graphite (0.34 nm), due to the introduction of oxygen-containing groups onto the GO sheets.24) In GO/ MnO2 precursor, the (001) reflection peak of layered GO disappeares, and several new peaks are displayed, which can be indexed to ¡-MnO2 (JCPDS 44-0141).25) The results indicate that the GO sheets have been covered by MnO2 layer. The T-rGO/MnO2 and V-rGO/MnO2 are similar to the GO/MnO2 precursor. However, the characteristic peaks ascribed to Mn3O4 (JCPDS 18-0803) are discovered in S-rGO/Mn3O4 or H-rGO/Mn3O4, suggesting that the phase transition occurs. This phenomenon could be explained that the reducing degree of GO and MnO2 have been greatly improved, with high energy microwave absorption and heat conversion.

Raman spectra can be used to gain more information about the structure of the hybrid. Figure 2 shows the Raman spectra of the GO/MnO2 material. The G band around 1590 cm¹1 and the D band around 1325 cm¹1are observed in the Raman spectrum of GO, corresponding to the E2g phonon of sp2 -bonded carbon atoms in a two-dimensional hexagonal lattice, as well as the defects and disorder carbon in the graphite layers. As for the GO/MnO2precursor, a distinct sharp peak located at 638 cm¹1appeared, which is ascribed to the Mn-O vibrations perpendicular to the direction of the MnO6 octahedral double chains of MnO2.26) In the rGO/MnO2 samples, the peak belonging to the Mn-O vibrations are well retained. On the other hand, the intensity ratio of the D to G bond (ID/IG) is generally accepted that theID/IGreflects the defect density of carbonaceous materials. For GO, theID/IG value is calculated as 0.94. In the rGO/MnO2samples,ID/IG values increase to over 1, revealing that the defects decrease after being reduced by chemical reaction. Typically, H-rGO/ Mn3O4possesses the largestID/IGvalue, indicating that the GO have been reduced greatest. The phase transformation from MnO2 to Mn3O4 (Fig. 1) also confirms the above analysis.

Fig. 1 XRD patterns of (a) GO, (b) GO/MnO2, (c) T-rGO/MnO2, (d)

V-rGO/MnO2, (e) S-rGO/Mn3O4and (f ) H-rGO/Mn3O4.

Fig. 2 Raman patterns of MnO2, GO, rGO, GO/MnO2, T-rGO/MnO2,

[image:2.595.320.532.69.243.2] [image:2.595.318.533.299.465.2]
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The composition is the most important and basic information of composite materials, and have been inves-tigated by TGA (Fig. 3). The weight loss values of GO/

MnO2, T-rGO/MnO2, V-rGO/MnO2, S-rGO/Mn3O4and H-rGO/Mn3O4 are calculated to be 15.5 mass%, 15.3 mass%, 15.2 mass%, 15.0 mass% and 14.9 mass%, respectively, which is almost consistent with the feeding ratio in the experimental section.

3.2 Morphological analysis

SEM images of different specimen are shown in Fig. 4. GO sheets decorated with nanoneedle MnO2 are clearly observed in Fig. 4(a). From Fig. 4(b)­(e), it can be clearly seen that there is a strong correlation between the reducing agents and the resulting nanostructure. There are no significant differences in the morphologies between GO/ MnO2 and T-rGO/MnO2. MnO2 nanoneedles are well attached without obvious damage, indicating that only GO are reduced to RGO. Besides the needle-like feather, the V-rGO/MnO2 occupies the nanoarchitecture with few nano-particles. The S-rGO/Mn3O4 consists of irregular-shape nanoparticles. To be noted, in H-rGO/Mn3O4 composite, the formation of uniform Mn3O4 nanoparticles of 20 nm has been found. This phase and morphology transition can be

Fig. 3 TG curves of GO/MnO2, T-rGO/MnO2, V-rGO/MnO2, S-rGO/

Mn3O4and H-rGO/Mn3O4.

[image:3.595.63.277.118.293.2] [image:3.595.114.481.346.776.2]
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explained by a“dissolution-precipitation”model. Firstly, the needle-like MnO2 is dissolved and dissociated into varied Mn-O units in the strong reducing agents. With the high energy microwave-assisted, the Mn-O units grow further into the Mn3O4 crystalline, which could easily aggregate into Mn3O4nanoparticles in order to decrease the surface energy. To our best knowledgment, the large specific surface area is favorable to the enhancement of electrochemical perform-ance. The BET values of all the samples are listed in Table 1, and the H-rGO/Mn3O4 has largest BET surface area of 149 m2/g.

3.3 Electrochemical properties

To explore potential applications in supercapacitors, the

GO/MnO2 samples are fabricated into work electrodes and characterized with CV and GCD measurements. From Fig. 5(a), it is found that the CV curves of the as-prepared specimens (especially for H-rGO/Mn3O4) have poor sym-metry with small redox peaks, which involve both surface adsorption of electrolyte cations (Na+) and proton (H+) incorporation27)as described in eq. (1):

MnO2þxNaþþyHþþ ðxþyÞe$MnOONaxHy ð1Þ

As we all know, the specific capacitance is mainly determined by the area under the CV curves. The H-rGO/ Mn3O electrode clearly exhibits the largest integrated area, which has an excellent electrochemical performance.

The GCD results of various electrodes are shown in Fig. 5(b). The samples of GO/MnO2, MnO2, rGO, T-rGO/ MnO2 and V-rGO/MnO2 have approximately triangular characteristics, which mean that they possess the relatively ideal capacitor behavior in a neutral aqueous electrolyte. While for the S-rGO/Mn3O4and H-rGO/Mn3O4, the asym-metrical characteristics can be clearly seen. The total capacitance could be attributed to the combination of the double-layer (of GO) and pseudocapacitive (of MnO2) contribution.28)Meanwhile, the discharge curves starts to be not straight because of the inner resistance (IR). The specific capacitances of the products are calculated from the discharging plot of the GCD curves by eq. (2).

[image:4.595.85.515.73.403.2]

C¼ ðItÞ=ðVmÞ ð2Þ

Table 1 BET surface area of the samples. Sample Specific surface area(m2/g)

GO 62

MnO2 36

rGO 88

GO/MnO2 79

T-rGO/MnO2 98

V-rGO/MnO2 110

S-rGO/Mn3O4 141

H-rGO/Mn3O4 149

Fig. 5 (a) CV curves at 10 mV/s; and (b) GCD curves at 0.5 A/g of GO/MnO2, MnO2, rGO, T-rGO/MnO2, V-rGO/MnO2, S-rGO/

[image:4.595.47.291.466.581.2]
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¦Vis the voltage range of one scanning segment,¦t is the time of a discharge cycle, and mis the weight of the active material. Herein, the specific capacitances C in eq. (2) is determined by the time of the discharge cycle. Based on eq. (2), the specific capacitance of the electrode is counted out and plotted in Fig. 5(c). The H-rGO/Mn3O4 electrode exhibits a specific capacitance as high as 324.9 F/g, which is superior to almost carbonaceous materials29)of 198 F/g and the pure MnO222) of 245 F/g. These results could be explained by the following aspects: (1) The doped MnO2 has been reduced to Mn3O4, and the diversity of ionic state of Mn3O4 is favorable to the enhancement of specific capaci-tance. (2) The mon-dispersed Mn3O4 with smaller size of 20 nm have the largest BET area of 149 m2/g. (3) The GO in H-rGO/Mn3O4is more efficiently reduced, and the electronic conductivity has been improved greatly.

Figure 6(a) shows the CV curves of the H-rGO/Mn3O4 composite electrode at different scan rates. At the lower scan rate of 10­50 mV/s, the CV curves exhibit an approximate rectangular shape with high symmetry. When up to 100 and 200 mV/s, some distortion has appeared, which is a normal behavior in most of material electrode.30,31)The GCD curves of H-rGO/Mn3O4composite at different current densities are shown in Fig. 6(b). It can be seen that all the curves are highly linear and symmetrical. However, the redox reaction really occurred in the electrode, which has been firmly confirmed in the amplified CV curve of Fig. 5(a). Thus, it can be deduced that the electrochemical property of the H-rGO/ Mn3O4 composite results from the combination of pseudo-capacitance along with the double layer capacity.

Cycle lifetime is one of the most critical factors in supercapacitor applications. A cyclic stability test of over 1000 cycles for H-rGO/Mn3O4 electrodes at a scan rate of 10 mV/s is performed ranging from 0.1 to 0.9 V. Figure 7 shows the specific capacitance retention as a function of cycle number. The electrode can retains 88%of initial capacitance after 1000 cycles, indicating that the H-rGO/Mn3O4 has a good long-term cycling stability.

4. Conclusion

In summary, the interesting phase and morphology

trans-formation in GO/MnO2 composite is discovered, with microwave-assisted reduction method. Especially, in hy-drazine hydrate system, the H-rGO/Mn3O4exhibits the best electrochemical performance with the specific capacitance of 324.9 F/g, and has good long-term cycle stability. These results are attributed to the diversity of ionic state, the large specific surface area, and the greatly improved electronic conductivity in H-rGO/Mn3O4. It is anticipated that the phase and morphology transformation resulted from the chemical reduction with the help of microwave radiation can be potentially applied in other transition metal oxides for energy conversion and storage.

Acknowledgements

The authors acknowledge thefinancial support by the Key discipline grant for composite materials from Shanghai Institute of technology (No. 10210Q140001), the Shanghai alliance project (LM201321). The authors thank the Shanghai Institute of Technology and East China Normal University.

Appendix

AFM images of as-exfoliated graphene.

Fig. 6 Electrochemical performance of H-rGO/Mn3O4composite electrode: (a) CV curves at different scan rates and (b) GCD curves at

different current density.

Fig. 7 Cycle performances of H-rGO/Mn3O4 composite at 10 mV/s in

[image:5.595.77.523.68.239.2] [image:5.595.315.531.286.445.2]
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Figure

Fig. 1XRD patterns of (a) GO, (b) GO/MnO2, (c) T-rGO/MnO2, (d) V-rGO/MnO2, (e) S-rGO/Mn3O4 and (f ) H-rGO/Mn3O4.
Fig. 3TG curves of GO/MnO2, T-rGO/MnO2, V-rGO/MnO2, S-rGO/Mn3O4 and H-rGO/Mn3O4.
Fig. 5(a) CV curves at 10 mV/s; and (b) GCD curves at 0.5 A/g of GO/MnO2, MnO2, rGO, T-rGO/MnO2, V-rGO/MnO2, S-rGO/Mn3O4 and H-rGO/Mn3O4 electrodes; (c) the value of specific capacitance calculated from (b).
Fig. 6Electrochemical performance of H-rGO/Mn3O4 composite electrode: (a) CV curves at different scan rates and (b) GCD curves atdifferent current density.
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References

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