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* E-mail: yzfang@chem.ecnu.edu.cn, pghe@chem.ecnu.edu.cn; Tel.: 0086-021-62233508; Fax: 0086-021-62233508 Received June 25, 2009; revised September 28, 2009; accepted November 10, 2009. Project supported by the National Natural Science Foundation of China (No. 20675031) and Shanghai Education Development Foundation (No.

2008CG30).

Chin. J. Chem. 2010, 28, 417—421 © 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 417

Enhancement of Electrochemical Capacitance of Carbon Nanotubes by Polythionine Modification

Xu, Ying(徐颖) Zhang, Xiaoyan*(张小燕) Wang, Yanzhi(王艳苓) He, Pingang*(何品钢) Fang, Yuzhi*(方禹之)

Department of Chemistry, East China Normal University, Shanghai 200062, China

Thionine molecules have been electropolymerized onto different types of carbon nanotubes (CNT) using a cy-clic voltammetry scanning technology, including multi-walled carbon nanotubes, single-walled carbon nanotubes and aligned carbon nanotubes. Results indicate that such prepared nanocomposites have combined the intrinsic far-adic capacitance of polythionine with the double layer capacitance of polythionine-CNT, and thus the polythionine modification obviously enhanced the CNT capacitance. Especially the carboxyl group modified multi-walled carbon nanotubes, which have made their nanotube tips opened, allowed much more electropolymerization cycles and then obtained a most significant increment in capacitance than the other three ones.

Keywords electrochemical capacitor, faradic pseudocapacitance, polymer, polymerization, carbon nanotube

Introduction

For their reversible and simple nature of the charge-storage process, short charging time, excellent cyclic life and high power density, the electrochemical capacitors (EC) have been developed as the third class of the energy storage devices, also called supercapaci-tors or ultracapacitors.1-4 Carbon, especially the carbon nanotube (CNT) is commonly used as EC double-layer capacitor material, due to its special high surface area and thus producing extremely high double-layer capaci-tance. Recently, conducting polymer materials have been investigated on their capacitance enhancement effect when they were combined with CNT.5,6 The ex-amples are polyaniline,7 poly(methyl methacrylate),8 polythiophene,9 polypyrrole, etc.10-12 Polythionine has excellent reversible and stable redox reactions owing to its electroactive heterocyclic nitrogen atoms, nitrogen bridges and free amino groups,13,14 thus showing prom-ising application potential as an ideal pseudocapacitor material to modify CNT. In this paper, polythionine has been studied on its capacitance enhancement effect for four different types of CNT. Owing to the highly re-versible redox properties of polythionine, and the spe-cial structural and electronic properties of CNT, the re-sulting polythionine-CNT nanocomposites displayed both high double layer capacitance and faradic capaci-tance. Results showed that the multi-walled carbon nanotubes with carboxyl groups (MWCNT-COOH) have allowed much more electropolymerization cycles

to carry out and thus increased capacitance value most significantly than the other three kinds of CNT, includ-ing the multi-walled carbon nanotubes without surface modification (MWCNT), single-walled carbon nano-tubes with carboxyl group modification (SWCNT-COOH) and aligned carbon nanotubes (ACNT).

Experimental

Thionine was purchased from Sigma, USA. Multi-walled carbon nanotubes (MWCNT, diameter 50 nm, length 1—2 µm) were prepared by a low pressure chemical vapor deposition method.15 The MWCNT with carboxyl group modification (MWCNT-COOH, diame-ter 40—60 nm, length 0.5—500 nm) were purchased from Shenzhen Nanotech Port Co. Ltd. Single-walled carbon nanotubes with carboxyl group modification (SWCNT-COOH, diameter＜2 nm, length 1—5 µm) were purchased from MicroTechNano, USA. These dif-ferent types of carbon nanotubes were separately dis-persed in DMF to 1 g/L. Then the corresponding CNT modified electrode was prepared by dropping 5 µL of the above CNT-DMF solution onto a glassy carbon electrode (GCE), and drying under an infrared lamp. The aligned carbon nanotubes (ACNT) with diameter 50—60 nm and length 7—8 µm were synthesized onto silica glasses by a low pressure chemical vapour deposi-tion method and then purified by treating with 20%—30% concentrated HNO3.

16 The ACNT electrode

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was prepared by linking the ACNT-silica glass with a GCE as working electrodes.17 Polymerization of thion-ine onto these different CNT electrode was carried out by using cyclic voltammetry scanning in 10 mL of CH3COOH solution (φ＝50%) containing 0.030 g of thionine and 0.1 mol/L KCl.

The polythionine-CNT composite nanomaterials were characterized by the electrochemical technologies of cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS), Fourier transform infrared spectroscopy (FTIR spec-troscopy), scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). All the electro- chemical experiments were carried out on a CHI 660 A electrochemical workstation using a three-electrode system. The CNT electrode was used as a working elec-trode, Ag/AgCl with KCl saturated as a reference elec-trode and Pt wire as a counter electrode. The FTIR spectroscopy measurements were carried out using a Nicolet Nexus 670 FT-IR spectrophotometer. The field emission SEM images were investigated by using a JEOL-S-4800 instrument. The XRD pattern was meas-ured on an X-ray Powder diffractometer of D8 AD-VANCE (Bruker AXS, German).

Results and discussion

Herein, thionine was electropolymerized onto four types of CNT, namely MWCNT, MWCNT-COOH, SWCNT-COOH and ACNT. The polymerization plot on the MWCNT-COOH electrode from －0.6 to 1.6 V was shown in Figure 1A, which displayed the gradually increased polymerization current and redox peak cur-rents of polythionine occurring at 0.8 and －0.4 V re-spectively. The other three CNT electrodes displayed the similar electropolymerization plots, in which the polymerization current was continuously increased with the CV scanning. It can be expected that during the electropolymerization process, the CNT electrode transferred electrons to oxidize the surrounding thionine monomers into their free radical ions, which were then linked together to form polymer film coating the nano-tube walls of CNT. Due to the special 3D structure of CNT providing a special large surface area for porous polymer to spread over, a continuous polymerization reaction has occurred. The resulting polythionine-CNT electrode was then investigated on its electrochemical properties by CV scanning in phosphate buffer solution (pH 7.0). As shown in Figure 1B, after polythionine modification, the MWCNT-COOH electrode gave a significantly enhanced double-layer capacitance current with well-defined redox peaks from polythionine films offering faradic pseudocapacitance. The other three polythionine-CNT electrodes have the similarly shaped CV plots with the enlarged double-layer capacitance current and obvious redox reaction peaks. The resulting

Figure 1 (A) thionine electropolymerization plots of the first 15 cycles from －0.6 to 1.6 V onto an MWCNT-COOH electrode. (B) the CV plots of MWCNT-COOH electrode in 0.1 mol/L PBS before and after thionine electropolymerization for 300 cycles at a rate of 0.1 V/s (PTh: polythionine). (C) capacitance increment fold vs. thionine electropolymerization cycle from －0.6 to 1.6 V for different CNT electrodes.

capacitance increment folds for the four kinds of CNT were then calculated by using the equation of C＝dq/ dE＝idt/dE＝i/v (i: the average current during the CV scan region, v: the CV scan rate) versus different poly-mer cycle numbers and compared in Figure 1C. As can be found, the capacitance enhancement for the different

Enhancement of Electrochemical Capacitance of Carbon Nanotubes

Chin. J. Chem. 2010, 28, 417—421 © 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 419

CNT electrodes was in the sequence of MWCNT- COOH ＞ ACNT ＞ SWCNT-COOH ＞MWCNT. The MWCNT electrode increased its capacitance at the elec-tropolymerization beginning and came to a platform at the 30th polymer cycle, while the other three electrodes raised their capacitance with much more electropoly-merization cycles, especially for the MWCNT-COOH electrode which continuously increased its capacitance until to the 250th polymerization cycle. The different capacitance enhancements can be explained by the following reasons. Firstly, the nanotube tips of MWCNT-COOH, SWCNT-COOH and ACNT have been opened, thus permitting much more polymeri- zation cycles with polymerization occurrence outside as well as inside the nanotubes than those in the MWCNT, whose nanotube tips closed actually decreased the polymerization efficiency. Additionally, as MWCNT- COOH and ACNT have more nanotube layers than SWCNT-COOH, the MWCNT-COOH electrode and ACNT electrode consequently carried out more polym-erization cycles and thus obtaining a higher capacitance increment than the SWCNTs-COOH electrode, which has only one layer of carbon nanotube for polythionine coating. For comparing the capacitance enhancement of MWCNT-COOH and ACNT by the polythionine modi-fication, it can be expected that as MWCNT-COOH existed in a disordered conformation, it permitted a more electropolymerization than the ACNT in a very ordered conformation, for the latter the polymerization was confined not only inside the individual carbon nanotube but also between every two aligned nanotubes.

As the polymer modification had the most signifi-cant enhancement for the capacitance of MWCNT- COOH electrode than the other three ones in the ex-periments, the electropolymerization of thionine was further investigated on the MWCNT-COOH electrode by SEM, IR, XRD and electrochemical technologies. As shown in the SEM images (Figure 2A), before electro-polymerization the primary MWCNT-COOH was dis-persed on the substrate with the individual nanotubes disordered and tangling with others, and with electro-polymerization cycling, more and more polymer aggre-gates appeared on the carbon nanotubes until very thick polythionine films were coated over all the carbon nanotubes, resulting in a composite with carbon nano-tubes inside and polymer outside. Additionally, the ca-pacitance value was continuously enhanced with the polythionine modification amount. The FTIR spectra of the resulting polythionie-MWCNT-COOH composite (Figure 2B curve c) show two typical peaks at 1730 and 1588 cm－1 respectively corresponding to ν(C＝O, COOH) and ν(C＝O, COO-), confirming the presence of COOH and COO－ groups (curve a) in the polythionine-MWCNT-

COOH material, and the ν(N—H) stretching vibrations at 3435 cm－1 and ν(C—H) stretching vibrations at 2960 cm－1 confirmed the presence of NH2 group-tagged polythionine (Curve b) on MWCNT-COOH. XRD pat-tern of the polythionine-MWCNT-COOH composite as shown in Figure 2C indicates an amorphous structure of polythionine in the composite, and the broad peak at about 20° comes from the underlying amorphous quartz substrate.18 EIS was then employed to further evaluate the electrochemical properties for the MWCNT-COOH electrode before and after electropolymerization (Figure 2D). As shown in the Nyquist plots, the semi-circle di-ameter was decreased from curve a to curve b after polythionine modification, indicating the decrease in the electronic transfer resistance of the electrode due to the polymer coating, which can be attributed to the electro-static adsorption between the positively charged polythionine on MWCNT-COOH and the negatively charged [Fe(CN)6]

3－/[Fe(CN)6]4－. The charge-discharge

behavior of the composite was examined by chronopo-tentiometry (CP) for multi-cycle scans. As shown in Figure 2E, before and after polythionine modification the MWCNT-COOH electrode both had a symmetry shaped charge-discharge curve, indicating its reversible charge-discharge reaction at a quick recharge rate and high cyclic efficiency without obvious charge-discharge memory. Additionally, as can be seen from the CP plot, the polythionine modification has enhanced CNT ca-pacitance by prolonging its charge and discharge time. By using the equation of C＝it/V, where i is the applied current, t the discharge time and V the voltage limit, it can be calculated that the MWCNT-COOH electrode has been significantly improved with its capacitance to 17 times after the polythionine modification, which was similar to the data (ca. 20 times) calculated by CV scanning in Figure 1C. The efficiency of discharge-to- charge (%) was then calculated by dividing the charging capacitance value by the discharging capacitance value, which respectively was around 100% and 98% for the MWCNT-COOH before and after polythionine modifi-cation, indicating the effective power release from the MWCNT-COOH and polythionine-MWCNT-COOH electrodes.

Conclusion

Electroactive polythionine has been electropolymer-ized onto CNT, and such prepared composite material exhibits both faradic capacitance and high double layer capacitance. Results showed that MWCNT-COOH with the nanotube tips opened permitted more electropoly-merization than other kinds of CNT, thus significantly enhancing the capacitance.

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Figure 2 (A) FESEM images for polythionine-MWCNT-COOH composite (from A1 to A4 with the electropolymerization of 0, 5, 50 and 300 cycles). (B) FTIR spectra for MWCNTs-COOH (a), thionine (b) and polythionine-MWCNT-COOH composite (c). (C) XRD pattern of the polythionine-MWCNT-COOH composite. (D) EIS plots of the MWCNT-COOH electrode before (a) and after polymeriza-tion (b), which was measured in 0.001 mol/L K3Fe(CN)6/K4Fe(CN)6 at 0.23 V. Frequency range: 105 to 10－2 Hz, amplitude: 0.005 V. (E) galvanostatic charge-discharge plots before (a) and after thionine polymerization (b) for MWCNT-COOH electrode. Cathodic and anodic currents: 0.2 mA, current density: 28.3 A/m2, high E limit: 0.8 V, low E limit: 0 V.

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