1. Introduction

 Cyclodextrins (CDs) are oligosaccharides composed of six, seven, or eight glucose units (α-, β- , or γ-CD respectively), which are toroidal in shape with a hydrophobic inner cavity and a hydrophilic exterior. These interesting characteristics can enable them to bind selectively various organic, inorganic and biological guest molecules into their cavities to form stable host-guest inclusion complexes or nanostructured supramolecular assemblies in their hydrophobic cavity.^[1]

 Carbon nanotubes (CNT) are a good candidate of electrodes for the electrochemical double layer capacitor (EDLC) since they have excellent electric conductivity and high aspect ratio, resulting in a good network or matrix for easy attachment of electrolyte ions in the EDLC.^[2,3] Composite of CNT and α-CD, therefore, could effectively enhance the conductivity of CD molecules in a film form and used for the electrode of electrochemical devices.

 We, however, found that the composite of CNT and α-CD has showed low capacitance though the specific capacitance retention was more than 80 % after 500 cycles, proving reliable cyclic stability up to 500 cycles.^[4] The hydrophilic CDs outside were easily detached from the hydrophobic surface of CNT so that CNTs themselves could only immobilize a few CD molecules.

 Chemically reduced graphene oxide (rGO) could be a solution for the improvement of capacitance and energy density since rGO could introduce the faradaic interaction by the reduction or oxidation from oxygen functional groups of GO^[5-8] and the hydrophilic rGO could immobilize the CD molecules effectively. Here we report synthesis of the composites made from α-CD and rGO in CNT matrix and physicochemical properties of the new composites. Capacitance is improved markedly up to 84 F/g with rGO at the current density of 0.7 A/ g compared with 2.6 F/g of the previous composites (CNT and α-CD only). The new composites electrodes show more redox processes as expected in the cycle voltammetry compared to the previous composites, presenting higher capacitances and energy densities.

2. Experimental

 20 mg of multi-walled carbon nanotube (20 um of length, 10 nm of diameter, Hanwha, Korea), 300 mg of Alpha Cyclodextrin (SIGMA-ALDRICH), and 40 mL of NaBH₄(SIGMA-ALDRICH) were used as purchased and put in a bottle together. 20 mg of GO was then added into the bottle. GO was synthesized as described elsewhere.^[8] The mixture was sonicated for 120 min to be dispersed well and then vacuum filtered slowly with the nylon filter (MILLIPORE, 0.2 um of pore size, 47 mm of diameter). Obtained new composite film was dried at 60℃ in a vacuum oven for overnight and investigated. We refer the new composite as CNT_CD_rGO and compare it with previous composite (CNT_CD) if needed.^[4]

 Scanning electron microscope (JSM 700F, JEOL, Japan) at different magnifications was performed to view the surface morphology of electrodes. Thermogravimetric analysis (TGA Instruments, Q600) was used to measure the component and weight of the elements. X-ray diffraction (XRD, Rigaku Rotaflex D/MAX System, Rigaku, Japan) at 40kV with Cu K_al(1.54Å) was characterized for the crystal structure of the electrodes.

Electrochemical measurements were carried out in two electrode cells with a separator and electrolyte of 1M H2SO4, using a Potentiostat/Galvanostat with impedance spectroscopy (Bio-Logic SAS, SP-150). The thin film was punched with 10.0 mm diameter holes for active electrodes, and the active electrode was put into a current collector (Ni foil, 125 μm, Aldrich). Cyclic voltammetry were performed with a scan rate of 10, 50, 200, and 500 mV/s, respectively. Galvanostatic charge and discharge measurement was carried at discharge voltage of 0.8 V with different current densities such as 0.5, 0.7, 1.0, 2.0, and 5 A/g, respectively.

3. Results and discussion

 Figure 1a shows XRD results of precursor graphite (PG), GO, α-CD, CNT, and the new composite (CNT_CD_rGO), respectively. Typical (002) peak of PG was shown near 26 degrees and the peak was shifted to around 10 degrees after becoming GO, indicating enhanced interlayer distance due to oxygen functional groups between layers. CNT shows the typical smooth graphitic peak near 26 degrees and the new composite have multiple peaks originated from GO, CD, and CNT. Figure 1b displays TGA results of CNT_CD and CNT_CD_rGO. The weight reduction of CNT_CD_rGO occurred at 200 and 3000C due to the decomposition of rGO (13.59 wt%) and CD (6.41 wt%), indicating successful inclusion of rGO and CD into the new electrode, and CNT_CD electrode shows 10.28 wt% of CD inclusion. CNT_CD_rGO included less CD compared with that of CNT_CD due to probably well reduction of rGO. It is noted that all data of CNT_CD electrode came from our previous results.^[4] SEM images of CNT_CD_rGO and CNT_CD electrodes are shown in Figure 2. Flat rGO was marked by the circles in Figure 2a and magnified in Figure 2b, revealing the presence of rGO on the surface of CNT_CD_rGO electrode, consistent with the results of XRD and TGA. Figure 2d shows real photo of the new composite film which is a free standing film form so it could be tailored easily for proper sized electrodes.

Figure 1. (a) XRD and (b) TGA results of samples.

Figure 2. SEM images of CNT_CD_rGO and CNT_CD samples (a-c) and the real photo of CNT_CD_rGO film (d).

 Figure 3 presents cyclic voltammogram (CV) of two electrodes. Maximum current of CV for CNT_CD_rGO electrode is bigger compared with that of CNT_CD electrode due to the Faradaic interaction originated from rGO. The typical EDLC behavior, a rectangular shape of CV presented in CNT_CD electrode in Figure 3b, was disappeared due to the redox interaction because of the oxygen functional groups existed in rGO. Galvanostatic charge/discharge measurements show distinct difference, different discharging time, of the two electrodes as shown in Figure 4a and 4c and the corresponding specific capacitance was plotted in Figure 4b and 4d. The specific capacitance can be calculated from the discharging curve using C = Q/(m * V ) = I_s/(m* dV/dt) where I_s is the constant supplied current corresponding to the current density, m is the average mass of one electrode, and dV/dt is the slope of the discharging curve [9]. The new electrode (CNT_CD_rGO) show more than 80 F/g of capacitance at 0.5 and 0.7 A/g, but the previous electrode (CNT_CD) has less than 4 F/g. The big enhancement of specific capacitance originated from the existence of rGO is demonstrated.

Figure 3. CV of (a) CNT_CD_rGO and (b) CNT_CD electrodes.

Figure 4. (a) Galvanostatic charge and discharge results of (a) CNT_CD_rGO film with (b) the corresponding specific capacitance. For comparison, the results of CNT_CD film (c, d) were taken from ref.^[4]

 Energy density as a function of power density, Ragone plot, is shown in Figure 5. The new electrode (CNT_CD_rGO) exhibited distinct improved energy density due to the Faradaic interaction based on rGO compared to that of the previous electrode (CNT_CD). The adding rGO into the CNT_CD composite, therefore, gives a positive effect on the electrochemical performance such as capacitance and energy density, suggesting a promising candidate of the electrode for electrochemical capacitors.

Figure 5. Ragone plot of CNT_CD_rGO film with reference of the CNT_CD electrode.

4. Conclusions

 New composite film was successfully synthesized from CNT, α-CD, and rGO. Capacitance is improved markedly up to 84 F/g compared to 2.6 F/g of the previous composites at the current density of 0.7 A/g. The improvement of capacitance was originated from rGO since rGO has oxygen functional groups, participated in the redox reactions. Following increased energy density of the new composite could be a promising candidate of the electrode for electrochemical capacitors.