Preparation and electrochemical properties of nitrogen-doped multi-walled carbon nanotubes

doi:10.1016/j.matlet.2010.08.081

Materials Letters

Volume 65, Issue 1, 15 January 2011, Pages 49-52

https://doi.org/10.1016/j.matlet.2010.08.081 Get rights and content

Abstract

High yield bamboo-shaped N-doped multi-walled carbon nanotubes (MWNTs), containing 2.27 at.% N, were fabricated by pyrolysis of pyridine at 800 °C in an Ar atmosphere. The electrochemical capacitive performances of this type of carbon fiber were evaluated in 6 M KOH aqueous solution. The specific capacitance of N-doped MWNT electrode is measured to be 44.3 F/g at the charge/discharge current of 1 mA, which is two times greater than that of the undoped one (19.9 F/g). The measurements also reveal that the N-doped MWNT electrode possesses lower equivalent series resistance (0.24 Ω) and higher knee frequency (1995 Hz) than the undoped one. This improvement is attributed to the unique morphology and electronic properties of N-doped MWNTs.

Introduction

Electric double-layer capacitors (EDLCs) (also called supercapacitors) are power sources that store energy mainly from the separation of electronic and ionic charges at the interface between the electrode materials and electrolyte solutions [1], [2], [3]. Various carbonaceous materials, such as graphene, carbon nanotubes, mesoporous carbon, carbon blacks, and activated carbons are often used as electrode materials in supercapacitors for their high specific surface areas (SSA) [4], [5], [6], [7], [8], [9], [10], [11]. Among them, carbon nanotubes are viewed as one of the most promising supercapacitor materials for their high accessible surface area, low electric resistance, low mass density, and high chemical stability [12], [13], [14], [15].

Generally, the performance of the supercapacitor depends strongly on the conductivity and SSA of the electrode materials. Therefore, fabrication of MWNTs with lower electric resistance and higher SSA is very important for the application. The synthesis of N-doped MWNTs provides a good opportunity to obtain a superior electrode material due to its enhanced conductivity [16]. Theoretical calculations have demonstrated that the substitutional nitrogen doping into MWNTs gives rise to metallic behaviors, because the nitrogen atoms provide additional free electrons for the conduction band [17]. The N-doped MWNTs are proved to have wide applications including field emitters, energy conversion, and energy storage [18]. Their enhanced conductivity is supposed to be an advantage for the application of supercapacitor.

In this paper, N-doped MWNTs were synthesized and characterized by transmission electronic microscopy (TEM). The electrochemical properties of undoped MWNTs and N-doped MWNTs were studied by the galvanostatic charge/discharge, cyclic voltammetry (CV), and AC electrochemical impedance spectroscopy. The electrochemical capacitive performances of N-doped MWNT electrode were compared with those of their undoped counterpart.

Section snippets

Experimental

Both undoped and N-doped MWNTs were synthesized by chemical vapor deposition using nano-Ni/bergmeal power as a catalyst. C₂H₄ was used as the carbon source, while pyridine was required to prepare N-doped MWNTs. The MWNTs were grown in an atmosphere of Ar/H₂ (100 sccm/40 sccm) with a deposition time of 30 min. To synthesize high yield MWNTs, the process temperature was set at 750 °C for undoped MWNTs and 800 °C for N-doped MWNTs. The initial productions, which consisted of catalyst and amorphous

Results and discussion

As shown in Fig. 1, N-doped MWNTs exhibit typical ‘bamboo-like’ compartments with diameters of ~ 40 nm. In contrast, undoped MWNTs are composed of several coaxial graphite cylinders with an average diameter of ~ 20 nm. The high resolution TEM (HRTEM) images show that the undoped MWNTs consist of straight graphitic layers along the tube axis, while N-doped tubes consist of periodically curved graphitic layers. The substitutional nitrogen doping can modify the morphology of MWNTs drastically by

Conclusions

In this work, bamboo-shaped N-doped MWNTs (with 2.27 at.% N content) were synthesized by pyrolysis of pyridine at 800 °C in an Ar atmosphere. The galvanostatic charge/discharge, CV and AC impedance spectroscopy were used to evaluate the electrochemical capacitive performance of carbon nanotube, using 6 M KOH as the electrolyte. It is proven that N doping can efficiently improve the electrochemical capacitance of MWNTs. The reason is that N-doped MWNTs possess larger SSA and pore volumes than the

Acknowledgments

This work is supported by the National Natural Science Foundation of China (Grant No.'s 50772018, 50402025), the Program for New Century Excellent Talents in University of China (NCET-07-0139).

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Thus, there have been many attempts made to improve the capacitance of CNT-based EDLCs, specifically by doping nitrogen and an additive of a metal oxide or a conducting polymer. Zhang et al. reported that N-doped MWNTs were synthesized with pyridine by chemical vapor deposition, with the specific capacitance of an N-doped MWNT electrode found to be 44.3 F/g [14]. Chen et al. prepared hybrid In2O3 nanowire/CNT films by a vacuum filtration method, reporting a specific capacitance of 64 F/g [15].
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Preparation and electrochemical properties of nitrogen-doped multi-walled carbon nanotubes

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgments

Solid State Ionics

J Electroanal Chem

Carbon

Chem Phys Lett

Mater Lett

Mater Lett

J. Pow Sour

Mater Lett

Chem Phys Lett

Electrochemical supercapacitors — scientific fundamentals and technological applications

Electrochemical supercapacitors: energy storage beyond batteries

Curr Sci