Program Title (English) ：Development of New Carbon Nanomaterials
利用者名（日本語） ：ウィナッダー ウォンウィリヤパン1), ウォラウット ムァンラット2)
Username (English) ：Winadda Wongwiriyapan1), Worawut Muangrat2)
所属名（日本語） ：1) キングモンクット工科大学ラドクラバン, 2) 信州大学
Affiliation (English) ：1) King Mongkut’s Institute of Technology Ladkrabang, 2) Shinshu University
The hybrid carbon nanostructure of nitrogen-doped graphene-grafted double-walled carbon nanotube (NG-DWCNT) was successfully synthesized by chemical vapor deposition (CVD). The specific capacitance of NG-DWCNT is 563 Fg-1, with only 5.65 % of degradation to initial specific capacitance after 5000 cycles, compared to that of graphene-grafted double-walled carbon nanotube (G-DWCNT) (348 Fg-1, 4.45 %) and double-walled carbon nanotube (DWCNT) (253 Fg-1, 4.35 %). NG-DWCNT hybrid nanostructure show superior specific capacitance and excellent electrochemical stability. The enhancement of electrochemical property was attributed to the high specific surface area, sharp-edged petal-like structure and nitrogen content of nitrogen-doped graphene-grafted on DWCNT bundle. These results suggest that NG-DWCNT hybrid nanostructure can potentially be candidate for high-performance energy storage device.
DWCNT was synthesized by floating catalytic CVD method at 1300 °C using ferrocene and thiophene dissolved in ethanol. The as-grown DWCNT was purified by acid and thermal treatments. Purified DWCNT was separately deposited by graphene and nitrogen-doped graphene (NG) by thermal CVD method at 1300 °C using ethanol and urea-ethanol, respectively. The morphology, nanostructure, crystallinity, purity and element composition of DWCNT and NGNS-DWCNT were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Raman spectroscopy and X-Ray photoelectron (XPS). Electrochemical properties were characterized by cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD).The specific capacitance was calculated from CV curves according to the following equation: CS = (I×Δt)/(ΔV×m), where CS is the specific capacitance (Fg-1), I is the applied current (A), Δt is the discharge time (s), ΔV is the voltage change after a full charge or discharge (v), and m is the mass of sample (g).
３．結果と考察（Results and Discussion）
The DWCNT assembled into the bundles with diameter in the range of 15-110 nm (data not shown). Fig. 1(a-b) shows FESEM images of G-DWCNT and NG-DWCNT, respectively. The petal-like structure graphene and NG obviously grafted directly on and along on the networked DWCNT bundle. TEM images in Fig. 1(c-d) confirmed the sharp-edge petal-like structure of graphene and NG grew on the sidewalls of DWCNT bundles, indicating that the hybrid carbon nanostructure. The size of graphene and NG are in the range of 19-168 and 15-120 nm, respectively.
Fig. 1 FESEM and TEM images of (a,c) G-DWCNT and (b,d) NG-DWCNT, respectively.
The Raman spectra consists of four characteristic peaks which are the radial breathing mode, D-, G-, and 2D-bands at 60-270, 1340, 1590 and 2670 cm-1, respectively (data not shown). The XPS survey spectra of DWCNT, G-DWCNT and NG-DWCNT consist of strong C1s peak at ~284 eV and O1s peak at ~532 eV. For NG-DWCNT, the N1s spectrum locate at ~400 eV (data not shown). The atomic percentage of NG-DWCNT is approximately 1.93 at.%.
Fig. 2 (a) CV curves of DWCNT, G-DWCNT and NG-DWCNT at scan rate of 5 mAs-1. (b) GCD curves at an applied constant current of 0.5 mA.
Fig. 2(a) shows the CV curves of all samples in the potential window from 0 to 1 V at a scan rate of 5 mVs-1. The quasi-rectangular CV curves are obtained for all samples. The CV curves of NG-DWCNT obviously exhibit two redox peaks in the range of 0.3 to 0.5 eV, demonstrating the characteristic feature of pseudocapacitance. Fig. 2(b) shows the GCD curves of all samples at an applied constant current of 0.5 mA. The NG-DWCNT shows a much longer discharge time than that of G-DWCNT and DWCNT, demonstrating that NG-DWCNT has a higher specific capacitance (563 Fg-1) than that of G-DWCNT (348 Fg-1) and DWCNT (253 Fg-1). The NG-DWCNT enabled 2.23- and 1.62-fold improvement in specific capacitance compared to G-DWCNT and DWCNT, respectively. The excellent electrochemical performance of NG-DWCNT is attributed to the high specific surface with sharp graphene edges and nitrogen doping in graphene. The cyclic stability of all samples were carried out by a GCD at the current of 0.5 mA for 5000 cycles (data not shown). The stability of specific capacitance of DWCNT, G-DWCNT and NG-DWCNT retains 95.65, 95.55 and 94.35 % after 5000 GCD cycles, implying that the excellent electrochemical stability.
 W. Muangrat, M. Obata, M. T. Htay, P. Dulyaseree, M. Fujishige, W. Wongwiriyapan, S. Morimoto and Y. Hashimoto, Hybrid carbon nanostructure of nitrogen-doped graphene-grafted double-walled carbon nanotube and their electrochemical properties (Under review).