Electrochemical oxidation of multi-walled carbon nanotubes and its application to electrochemical double layer capacitors

doi:10.1016/j.elecom.2005.01.008

Electrochemistry Communications

Volume 7, Issue 3, March 2005, Pages 249-255

https://doi.org/10.1016/j.elecom.2005.01.008 Get rights and content

Abstract

The effect of electrochemical oxidation in 0.2 M HNO₃ for multi-walled carbon nanotubes (MWNTs) on the performance of electrochemical double layer capacitors (EDLCs) was studied. Scanning electron microscope and transmission electron microscope images reveal that electrochemical oxidation increases the specific area of MWNTs by cutting off the nanotube tips. Cyclic voltammetry and constant current charging/discharging was used to characterize the behavior of EDLCs of the oxidized-MWNTs in 1.0 M H₂SO₄. The specific capacitance of the oxidized-MWNTs was remarkably improved. The maximum specific capacitance of 335.2 ± 13.1 F g⁻¹ (n = 3) was obtained for the oxidized-MWNTs with open tips, which is 11 times than that of normal MWNTs (32.7 ± 7.1 F g⁻¹). Electrochemical oxidation is hence an effective way to improve the performances of the MWNTs electrodes in EDLCs application.

Introduction

Electrochemical capacitors are charge-storage devices that have the characteristics of high energy density, great power density and long cycle life [1]. According to the energy storage mechanism, electrochemical capacitors can be divided into two types: electrochemical double-layer capacitors (EDLCs) and redox supercapacitors. EDLCs is being considered for a variety of applications such as the capacitive deionization of water and as pulse power sources for cellular devices [2], [3]. Three kinds of materials have been studied for the development of electrochemical capacitors: (1) carbon having very high surface area [4], (2) certain metal oxides, and (3) conducting polymers [5], [6], [7].

Carbon nanotubes possess many special properties, such as open mesoporous structure, high electrical conductivity and chemical stability as well as extremely high mechanical strength and modulus [8], [9], [10], [11], [12], [13]. These properties, which not only help in the transportation of ions but also facilitate the charging of the double layer, will exhibit advantageous attributes in the developments of electrochemical capacitors [14]. Both single and multi-walled carbon nanotubes (SWNTs and MWNTs, respectively) have been recognized as a potential electrode material for electrochemical capacitors [15], [16], [17], [18], [19], [20], [21], [22], [23].

To improve the capacitance values of carbon nanotubes, several methods have been proposed, including: (a) the chemical treatment (KOH) for activation to increase the micropore volume of nanotubes [24]; (b) the forming of the MWNTs-conducting polymers composites such as polypyrrole-MWNTs [25], [26] and SWNTs-polyaniline polymer composites [14]; and (c) the functionalization of nanotubes with transition metal oxides [27], [28], [29].

We have successfully synthesized high-density well-aligned carbon nanotubes, which are multi-walled and vertically aligned on a large area of substrates, such as Ta [30], [31], [32]. These MWNTs with a high electrochemically accessible surface area, high electrical conductivity, and useful mechanical properties [33], have potential applications as electrode materials for developing sensors [34], [35], [36], [37] and supercapacitors [38]. Electrochemical oxidation (etching) of MWNTs in 0.1 M KCl at potentials more positive than 1.7 V vs. Ag|AgCl (3 M NaCl) has been reported [39]. Electrochemical oxidation of CNTs represents a new method for controlling the length and width of CNTs, which can be used for the development of nanotubes-based nanomaterials [40]. In this paper, we report the electrochemical oxidation of the MWNTs by potential cycling in HNO₃ between +1.0 and +2.0 V. The caps of nanotubes are opened during electrochemical oxidation. As a result, the specific capacitance of the MWNTs after electrochemical oxidation is 11 times higher than that of normal MWNTs. Electrochemical oxidation is hence a simple yet effective way to improve the performances of the MWNTs electrodes in EDLCs application.

Section snippets

Synthesis and electrochemical oxidation of well-aligned MWNTs

Detailed description on the synthesis of well-aligned MWNTs was reported previously [30], [31], [32]. In this work, a Ta-plate was used as a substrate. It was first coated with a thin cobalt (Co) layer of 8–50 nm as the catalyst by magnetron sputtering. The nanotubes with diameters in the range of 80–200 nm and a length of about 10 μm can be routinely obtained depending on the Co layer thickness and growth time [30]. The typical scanning electron microscope (SEM) image of MWNTs before and after

Instrumentation

Electrochemical measurements were performed using CHI 660A electrochemical workstation (CH Instruments Inc., USA) in a three-electrode arrangement, including a working electrode (MWNTs electrode or glassy carbon electrode), a platinum counter electrode and a 3 M KCl–Ag|AgCl reference electrode. All potentials were quoted vs. the 3 M KCl–Ag|AgCl reference electrode. The experiments were performed at room temperature (≈25 °C).

SEM and transmission electron microscope (TEM) study of MWNTs was carried

Electrochemical oxidation of MWNTs in 0.2 M HNO₃

Electrochemical oxidation of carbon nanotubes has been studied recently [39], [40]. Crooks and co-workers [39] reported that MWNTs can be etched at potentials more positive than 1.7 V vs. Ag|AgCl (3 M NaCl) in an aqueous 0.1 M KCl electrolyte solution. The logarithm of etching rate was proportional to the applied electrode potential, suggesting a kinetic etching mechanism. TEM images of etched MWNTs indicated that (1) etching began at the distal end and proceeded preferentially along the

Conclusion

Electrochemical oxidation of MWNTs increases the specific area of nanotubes by cutting off the nanotube tips and by converting the surface property of nanotubes from the hydrophobic state to the hydrophilic state. The electrochemical oxidized-MWNTs have improved specific capacitance, which is 11 times than that of normal MWNTs (32.7 ± 7.1 F g⁻¹). The enhanced capacitance and improved stability of the oxidized-MWNTs electrode indicates that electrochemical oxidation is a simple and effective way to

Acknowledgements

This work was supported by Academic Research Grant of the National University of Singapore R-377-000-015-112 and R-398-000-006-112 to F.-S.S. and R-154-000-118-305 to T.M.L.

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¹: These authors contributed equally.

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Electrochemical oxidation of multi-walled carbon nanotubes and its application to electrochemical double layer capacitors

Abstract

Introduction

Section snippets

Synthesis and electrochemical oxidation of well-aligned MWNTs

Instrumentation

Electrochemical oxidation of MWNTs in 0.2 M HNO3

Conclusion

Acknowledgements

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Chem. Phys. Lett.

Carbon

Carbon

Thin Solid Films

Electroanalysis

Electrochem. Commun.

J. Electroanal. Chem.

Synth. Metals

Electrochem. Commun.

J. Electroanal. Chem.

Sensor. Actuat. B

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Electrochemical Supercapacitors

J. Electrochem. Soc.

Electrochemical oxidation of MWNTs in 0.2 M HNO₃