Lei Zhou, Dawei He*, Honglu Wu, Zenghui Qiu

Transcription

1 Synthesis of Three Dimensional Graphene/Multiwalled Carbon Nanotubes Nanocomposites Hydrogel and Investigation of their Electrochemical Properties as Electrodes of Supercapacitors Lei Zhou, Dawei He*, Honglu Wu, Zenghui Qiu Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing , PR China (Lei Zhou). (Dawei He*). Keywords: supercapacitor, graphene, Carbon Nanotubes, RGO/MWCNTs, hydrogel, electrochemical performance. Abstract. A facile synthesis method of three dimensional reduced graphene oxide (RGO)/multiwalled carbon nanotubes (MWCNTs) hydrogel was introduced. Hydrogel samples which were characterized by scanning electron microscope(sem), transmission electron microscopy (TEM) and examined by X-ray diffraction (XRD) have been used as the electrode of supercapacitor. Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) were used to investigate the Supercapacitors which we have fabricated. Because MWCNTs inserting into layers of RGO homogeneously prevent the layers of RGO from stacking and enlarge the specific surface area of graphene, the specific capacitance of RGO/MWCNTs material has been greatly improved. At the current density of A/g, the specific capacitance of RGO/MWCNTs electrode is about 176F/g, which means a 52% increasement compared to which of pure RGO material electrode. And the specific capacitance of RGO/MWCNTs also achieves a good rate property. Introduction In recent decades, supercapacitor has been used as an energy storage device and has gained world wide attentions. Comparing with conventional capacitors, it has many advantages - larger capacity, higher powerdensity, faster process of charging and discharging, no environmental pollution, etc. Therefore, supercapacitor is a energy storage device which is highly efficient, functional, and environmentally friendly. With different charge storage mechanism, supercapacitors can be categorized to electrochemical double-layer capacitance (EDLC)[1-2] and pseudo-capacitance. An electrochemical double-layer capacitance, e.g. carbon-based supercapacitors[3-6], its double-layer capacity is generated from the charge separation of electrodeand electrolyte. Pseudo-capacity is generated from the highly reversible chemisorption, desorption or redox reaction of electroactive species on the surface of the capacitor. Supercapacitors prepared with transition metal oxides, e.g. RuO2, MnO2[7-9], etc, and conductive polymers[10-11], e.g. polyaniline, polypyrrole, etc, are based on the principle of Faraday Pseudocapacitance. Graphene and carbon nanotubes are known as carbon allotropes in materials science. Graphene are films of single-layer carbon atoms stripped out from graphite materials. They are two-dimensional alveolate crystals which are formed with single-layer hexagonal carbon atoms, and this strict two-dimensional structure will introduce advantages like high specific surface area, excellent crystal properties and electrical properties. Based on the above characteristics, graphene are widely applied in photocatalysis, electrochemical, electronics, optoelectronics, and biochemical sensor field. One-dimensional carbon nanotube of hexagon structure has many advantages of light weight, high strength, and strong toughness, etc, and many excellent mechanical, electrical, physical and chemical properties. Carbon nanotube is widely used in supercapacitor and superconductivity, etc.

2 Experiment Figure 1. (a) 3D RGO/MWCNTs hydrogel. (b) Coin-size supercapacitor cells 1. Synthesis of 3D RGO/MWCNTs hydrogel Graphite oxide (GO) was prepared from natural flake graphite by the well-known Hummers method[12-13]. The synthesis method of 3D reduced graphene oxide (RGO)/multiwalled carbon nanotubes (MWCNTs) hydrogel is as follows: First, a certain amount of GO is added to deionized water with ultrasonication for about 40min. Second, adding MWCNTs into GO solution for 30min ultrasonication to form uniform GO-MWCNTs hybrid solution. Third, adding the right amount of reducing agent NaHSO3[14] into the mixed solution which is prepared previously and then stirring for about 10min. The suitable concentration of NaHSO3 is approximate 30-50mmol/L. Finally, the solution is poured into a reaction kettle and put into drying oven for about 10h at temprature of 120. Then three-dimensional RGO/MWCNTs hydrogel (Fig. 1a) could be obtained. 2. Fabrication of coin-size supercapacitor cells The obtained RGO/MWCNTs hydrogel was washed 5-6 times by deionized water and put into Vacuum freeze drying oven. After about 12h we took out the hydrogel from the Vacuum freeze drying oven and cut the hydrogel into 1.2mm thick slices with ablade as the electrode of supercapacitor, the nickel foam was used as the current collector, and thin polytetrafluoroethylene film was used as the separator. All of them were soaked in 6mol/L KOH aqueous solution for 8h. Then the coin-size supercapacitor cells (Fig. 1b) were assembled with the RGO/MWCNTs electrode, nickel foam current collector and polytetrafluoroethylene film separator. 3. Characterization of RGO/MWCNTs and investigation of the cells The RGO/MWCNTs samples were characterized and examined by scanning electron microscope(sem, HITACHI S-4800), transition electron microscopy (TEM, JEOL JEM-1400) and X-ray diffraction (XRD, Bruker D8 ADVANCE using Cu Κα radiation). The cells were investigated by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD). Results and discussion 1. Characterization

3 Intensity (a.u.) GO RGO Theta (degree) Figure 2. XRD patterns of GO and RGO. Fig. 2 shows the XRD patterns of GO and RGO. From the XRD pattern of GO, we can see only a sharp and intense peak at around 2θ=11, which indicates that the purity of GO is very high and the interlayer distance has increased to appropriate nm. the XRD pattern of RGO exhibits that diffraction peak at 2θ=11 of samples which has been processed by Hydrothermal method at 120 has become weak, and then a new diffraction peak of RGO appears at 2θ= 24. This phenomenon shows that in the hydrothermal process, Oxygen containing functional groups in samples are gradually decomposed so that diffraction peak moves to high degree. Hydrothermal treatment makes the crystal structure of GO gradually changing to the crystal structure of RGO. The XRD pattern of RGO shows that our method of fabricating the 3D hydrogel is indeed feasible. Figure 3. Scanning electron microscope(sem) images of RGO at Different magnifications.

4 Current (A) Figure 4. (a)transmission electron microscopy (TEM) image of RGO. (b)transmission electron microscopy (TEM) image of RGO/MWCNTs. F ig. 3 shows the typical SEM images at different magnifications of RGO. In the reduction stage, porous 3D interconnected network forms in the hydrogel gradually as the images show. And the pore size ranges from 2μm to 10μm. Fig. 4a, Fig. 4b show the TEM images of 3D RGO hydrogel and 3D RGO/MWCNTs hydrogel respectively. In figure 4a, RGO is still expressed as a form of fold transparent gossamer, and edge portion of RGO rolls up and overlaps. And from figure 4b, we can clearly see that MWCNTs acted as spacer insert into layers of RGO homogeneously. 2. Electrochemical behavior a 0.10 b 5 0 A/g 0.5A/g 1A/g 2A/g 5A/g V/s 2V/s 5V/s 0.1V/s V/s 0.5V/s Figure 5. (a) CV curves of pure RGO electrode at different scan rate. (b) GCD curves of pure RGO electrode at different current density. CV and GCD are both employed to investigate the electrochemical performance of samples including both pure graphene and RGO/MWCNTs composite electrode. Fig. 5a presents CV curves of pure RGO electrode at different scan rate from 10mV/s to 200mV/s. The specific capacitance of pure RGO can be calculated with the equation C = A/sm V, where A is the area of the cv curve, s is the potential scan rate, m is the mass of single electrode material, and V is the potential window. Fig. 5b shows the GCD curves of pure RGO electrode at different current density from A/g - 5A/g. On the basis of GCD curves, the specific capacitance also can be calculated with equation C= 2I t/m V, where I is the charge/discharge current, t is the discharge time, m is the mass of single

5 Current (A) electrode material, and V is discharge potential. At different current densities of A/g, 0.5A/g, 1A/g, 2A/g, 5A/g, the specific capacitances of pure RGO are about 112F/g, 104F/g, 100F/g, 92F/g, 86F/g respectively. a b A/g 0.5A/g 1A/g 2A/g 5A/g V/s 2V/s 5V/s 0.1V/s V/s 0.5V/s Figure 6. (a) CV curves of RGO/MWCNTs (ratio ) electrode at different scan rate. (b) GCD curves of RGO/MWCNTs (ratio ) electrode at different current density. Fig. 6a, Fig. 6b show the CV and GCD curves of RGO/MWCNTs (ratio ) electrode. On the basis of GCD curves, the specific capacitances of RGO/MWCNTs electrode are respectively 176F/g, 164F/g, 140F/g, 128F/g, 120F/g at different current density A/g, 0.5A/g, 1A/g, 2A/g, 5A/g. Comparing to pure RGO electrode of 112F/g at current density of A/g, the specific capacitance of RGO/MWCNTs () material is 176F/g which achieves an increase of 52%. And even at a high current density of 5A/g, the specific capacitance of RGO/MWCNTs composite material still remains higher than 120F/g. The results show that MWCNTs inserting into layers of graphene prevent graphene layers from stacking, enlarge the surface area of graphene, and increase the electrical conductivity of the electrode, so the specific capacitance of RGO/MWCNTs electrode has been greatly improved, and also has a good rate property. a b Figure 7. (a) GCD curves of weight ratio of and (RGO to RGO/MWCNTs) at current density 1A/g. (b) GCD curves of weight ratio of and (RGO to RGO/MWCNTs) at current density 2A/g. Fig. 7a, Fig. 7b present the GCD curves of weight ratio of and at current density 1A/g and 2A/g respectively. In the discharge stage, voltage drop of hydrogel is about 4V and 6V at current density of 1A/g and 2A/g, while voltage drop of hydrogel is both only 2V. In most cases, voltage drop of RGO/MWCNTs electrode are almost less than which of pure RGO electrode. Because of addition of MWCNTs, conductivity of composite hydrogel has increased and resistance of composite hydrogel has decreased.

6 Specific capacitance (F/g) :1 4:1 3:1 2: Current density (A/g) Figure 8. Line chart of specific capacitance of different ratio(rgo to MWCNTs) at different current density. As fig. 8 shows, with the increase of current density, all the specific capacitance of different ratio decrease linearly. This indicaties that specific capacitance of 3D hydrogel which prepared by our method has a good rate property. 5:1 4:1 3:1 2: Figure 9. GCD curves of different weight ratio of GO to MWCNTs at a current density of A/g. Fig. 9 presents the GCD curves of different weight ratio of RGO to MWCNTs at a current density of A/g. From the curves, the specific capacitances of different weight ratio (RGO to MWCNTs), 5:1, 4:1, 3:1, 2:1, are 116F/g, 108F/g, 137F/g, 140F/g, 160F/g, 176F/g respectively. As the content of MWCNTs increases, the specific capacitance of composites increases as well. But when the content of MWCNTs increases to weight ratio 1:2 of RGO to MWCNTs, the composite hydrogel is very fragile so that the hydrogel cannot be cut into slices and cannot be used as electrode directly.

7 So the weight ratio (RGO to MWCNTs) is the better ratio to achieve a better electrochemical performance. Conclusions This paper illustrates a simple and environment friendly method for the synthesis of 3D RGO/CNTs hydrogel, and an electrochemical test is conducted to investigate the performance of supercapacitor electrode made of 3D RGO/CNTs. The experiment results show that the specific capacitance of the RGO/CNTs electrodes is not only much higher than the specific capacitance of pure RGO, but also has a good rate property. On the other hand, CNTs distribute in layers of RGO uniformly, which acts as spacer. This increases the specific surface area of the composite electrode, and increase the conductivity of the electrode material as well. RGO/CNTs Composite material with a 3D structure remedy the defects of one-dimensional CNTs and two-dimensional graphene, and this may make RGO/CNTs material gain wide applications. Acknowledgements We acknowledge supports from the National Basic Research Program 973 of China (Grants No. 2011CB and No. 2011CB932703), Chinese Natural Science Fund Project (Grants No , No , and No ), and Beijing Natural Science Fund Project (Grant No ). References [1] M. Winter, R.J. Brodd, Chem. Rev. 104 (2004) [2] A. Burke, J. Power Sources 91 (2000) [3] Wang Y, Shi ZQ, Huang Y, Ma YF, Wang CY, Chen MM, ChenYS. JPhys Chem C 2009;113(30): [4] Futaba DN, Hata K, Yamada T, Hiraoka T, Hayamizu Y, Kakudate Y, Tanaike O, Hatori H, Yumura M, Iijima S. Nat Mater 2006;5(12): [5] Frackowiak E, Beguin F. Carbon 2002;40(10): [6] Stoller MD, Park SJ, Zhu YW, An JH, Ruoff RS. Nano Lett 2008;8(10): [7] Mao L, Zhang K, Chan H S O, et al. Journal of Materials Chemistry, 2012, 22(5): [8] Yu G, Hu L, Liu N, et al. Nano Letters, 2011, 11(10): [9] Yousefi T, Golikand A N, Mashhadizadeh M H, et al. Current Applied Physics, 2012, 12(2): [10] Wang D W, Li F, Zhao J, et al. Acs Nano, 2009, 3(7): [11] Zhang D, Zhang X, Chen Y, et al. Journal of Power Sources, 2011, 196(14): [12]W. S. Hummers and R.E.Offenman, Journal of the American Chemical Society, 80, (1958). [13] Y.X.Xu, L.Zhao, H.Bai, W.J.Hong, C.Li and G.Q.Shi, Journal of the American Chemical Society, 131, (2009). [14] Chen W, Yan L, Bangal P R. The Journal of Physical Chemistry C, 2010, 114(47):