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0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2009.08.026
nanorods grown on CNTs were really comprised by RuO2, Raman spectroscopy was employed to analyze. The capacitance measure-ment used Ag/AgCl as the reference electrode and Pt was the counter electrode. The electrolyte was KOH of the different concentration from 0.01 M to 6.0 M. Furthermore, the character-istics of charge–discharge stability of EDLC were examined.
3. Results and discussion
Fig. 1 shows the SEM and TEM images of pristine CNTs.
Fig. 1(a) reveals the CNTs were vertically aligned and uniform on the SUS substrate surface. The length of CNTs was approximately 80
m
m.Fig. 1(b) is the enlarged image of CNTs. The vertically aligned CNTs were shown with a high number density of about 1.2 109cm 2.Fig. 1(c) shows the TEM image of CNTs. The CNT exhibited a hollow and multiwall structure, and the diameter was about 35 nm. The inset image ofFig. 1(c) shows the selected area electron diffraction pattern of a CNT. The (002) and (004) planes of graphite pattern were observed clearly. It revealed that the CNT crystallized very well.Fig. 2shows the SEM and TEM images of RuO2/CNT composites.
FromFig. 2(a), we found that the RuO2nanorods were in needle-like shape and deposited on CNTs uniformly.Fig. 2(b) shows the TEM image of RuO2 nanorods grown on CNTs. The CNT was the principle part and template. The average length of RuO2nanorods were about 300 nm and the average diameter was about 6 nm. The layers of the CNTs were not observed, because the CNTs were covered with the RuO2nanorods.Fig. 2(c) shows the enlarged TEM image of RuO2nanorod. The RuO2crystallized very regularly and the growth direction was along the [001] direction. The inset image ofFig. 2(c) shows the selected area diffraction of RuO2nanorods [21]. The continuous lattice indicated that the nanorod was single crystal which is in agreement with the result drawn from the inset inFig. 2(c).
In order to confirm the nanorods were RuO2, the Raman spectroscopy was used to analyze the composing.Fig. 3shows the Raman spectra of the pristine CNTs and RuO2/CNT composites, respectively. The solid line shows the Raman spectra of pristine CNTs. The pristine CNTs have D-band at 1334 cm 1and G-band at 1571 cm 1. The D-band and G-band indicates the disordered Fig. 1. (a) Cross-sectional SEM image of the vertically aligned CNTs on SUS substrate, (b) the enlarged image of CNTs, (c) TEM image of CNTs and the selected area electron diffraction pattern.
Fig. 2. (a) SEM image of RuO2nanorods grown on CNTs, (b) TEM image of the needle-like RuO2nanorods grown on CNT, (c) the enlarged TEM image of RuO2/CNT composites and the selected area electron diffraction pattern.
carbon and crystalline graphite, respectively[22]. The dash line shows the Raman spectra of RuO2/CNT composites. It shows the D-band and G-D-band of CNTs, and the Egat 517 cm 1, A1gat 629 cm 1, and B2gat 695 cm 1of RuO2[23].
Fig. 4shows the capacitance–concentration curves of pristine CNTs and RuO2/CNT composites in KOH electrolyte. The solid line shows the curve of pristine CNTs, and the dash line shows the curve of RuO2/CNT composites. It was found when the KOH electrolyte concentration was higher then 0.5 M, the capacitance increased linearly and slowly. On the other hand, when the KOH electrolyte concentration was lower than 0.5 M, the capacitance declined sharply with the decreasing concentration. The ions motion in the electrolyte affects the capacitance of EDLC. In this study, when the electrolyte concentration was lower than 0.5 M, with the increase of ion concentration, the effective ions for enhancing the capacitance increased. On the other hand, when the electrolyte concentration was higher than 0.5 M, the effective contributed ions, which could enhance the capacitance, in the electrolyte were saturated. Therefore, the superfluous ions in the electrolyte could not remarkably enhance the capacitance. The capacitance–concentration curves had a transition point at 0.5 M.
Furthermore, with the same electrolyte concentration, the
capacitance of RuO2/CNT composites was much higher than that of the pristine CNTs. Because the RuO2formed with not only the EDLC but also the pseudocapacitor. In 0.5 M KOH, the capacitances of pristine CNTs and RuO2/CNT composites were 0.47 F/g and 5.21 F/g, respectively.
Fig. 5(a) shows the charge–discharge curve of RuO2/CNT composites which was measured after 500 cycles. The curve was a very regular triangle wave. This means that the RuO2/CNT composites demonstrated a stable capacitance characteristic. The capacitance of RuO2/CNT composites increased a little for being measured for 500 cycles. It would be that when we started to measure the charge–discharge curves, the electrolyte did not contact the electrode surface completely. But after 500 cycles measurement, the electrolyte flow into the CNTs, resulting in increasing the interface area between the electrolyte and electrode. Therefore, the capacitance had a little increase through the 500 cycles measurements.
4. Conclusion
We synthesized the RuO2 nanorods on CNTs. The needle-like RuO2 nanorods has played an important role in enhancing the capacitance. There is a transition concentration of KOH electrolyte in this study. When the concentration of KOH electrolyte was higher than 0.5 M, the capacitance could not increase remarkably.
The RuO2/CNT composites demonstrated a stable cycle life. The capacitance stability made the RuO2/CNT composites to be a promising supercapacitor device material.
Fig. 3. Raman spectra of CNTs and RuO2/CNT composites.
Fig. 4. Capacitance–concentration curves of pristine CNTs and RuO2/CNT composites in KOH electrolyte.
Fig. 5. (a) The charge–discharge curve of RuO2/CNT composites after being measured for 500 cycles and (b) the initial charge–discharge curve versus the curve after being measured for 500 cycles of RuO2/CNT composites.
Acknowledgments
This work was partly supported by the National Science Council of Taiwan under contract number NSC-96-2221-E-011-078. The authors would like to acknowledge Prof. Wen-Chang Yeh and Prof.
Liang-Chiun Chao for their valuable discussions and assistance in measurements.
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