Highly dispersed Pd nanoparticles on 2-aminophenoxazin-3-one functionalized MWCNTs surface for methanol electro-oxidation in alkaline media

doi:10.1016/j.mseb.2010.03.082

Materials Science and Engineering: B

Volume 171, Issues 1–3, 25 July 2010, Pages 109-115

https://doi.org/10.1016/j.mseb.2010.03.082 Get rights and content

Abstract

A simple and rapid synthesis method to prepare well-dispersed Pd nanoparticles on 2-aminophenoxazin-3-one (APZ) functionalized multi-walled carbon nanotubes (Pd/APZ-MWCNTs) is presented. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transforms infrared spectrometry (FT-IR) as well as Raman spectrum are used for the catalysts characterization. The electrocatalytic properties of the Pd/APZ-MCWNTs catalyst for methanol oxidation have been investigated by cyclic voltammetry and chronoamperometry methods. In contrast to the Pd nanoparticles supported on acid-treated MWCNTs and carbon black catalysts, the electrochemical results demonstrate that Pd/APZ-MCWNTs exhibit better electrocatalytic activities and stability, mainly due to the uniform dispersion and small particle size of Pd nanoparticles on the APZ-MWCNTs supports. The results imply that the Pd/APZ-MWCNTs catalyst shows the better electrocatalytic performances and has a promising application potential in fuel cells.

Introduction

Direct methanol fuel cells (DMFCs) are recognized to be a promising power source for portable electronic devices and electric vehicles [1], [2]. However, commercialization of these fuel cells has been facing serious difficulties due to kinetic constraints of the methanol oxidation reaction (MOR). Pt-based metals and alloys are the best electrocatalysts for the MOR in acid solutions [3]. However, the high cost and limited supply of Pt constitute a major barrier to the development of DMFC. It is therefore desired to develop low-cost non-platinum electrocatalysts with comparable or improved kinetics for anodic methanol oxidation. Many non-platinum metals have recently been studied in alkaline medium [4], [5], among them, Pd is a suitable low-cost transition metal and its abundance is at least fifty times more than that of Pt on earth [6], and the results reveal that Pd is a good electrocatalyst for methanol and ethanol oxidation in alkaline media [7], [8], [9]. Additionally, Masel and co-workers [10] have disclosed that Pd and Pd/C catalysts can overcome the CO-poisoning effect and thereby yield high performance in DMFCs. Thus, Pd-based catalysts seem to be a promising anode in alkaline DMFCs. However, the activity and stability of the Pd catalyst are still in need of improvements.

Recently, carbon nanotubes (CNTs) have captured the attention of researchers worldwide due to their unique properties, such as high external surfaces, good electronic conductivity, large surface to volume ratio and high stability, which make CNTs an ideal supporting material [11], [12]. However, realistic applications have been hindered by difficulties associated with processing. In principle, metal nanoparticles are spontaneously formed at the defect sites on the surface not on sidewall of CNTs [13]. Therefore to obtain the homogeneously dispersed nanoparticles, the surface of CNTs must be modified via a proper functionalization. Generally speaking, this may be done either by covalent [14], [15], [16] or by noncovalent interactions [17], [18].

We have proposed new methods of preparing 2-aminophenoxazin-3-one monolayer modified MWCNTs supported PtRu catalysts by chemical method which are active towards methanol electro-oxidation in acid solution at room temperature [19], as part of our continuing work on developing low-cost processes for the preparation of homogeneous and well-defined nanoparticle catalysts for fuel cell applications. In this paper, APZ-functionalized MWCNTs supported Pd nanoparticles for methanol electro-oxidation in alkaline media has been detailed studied. The enhanced electrocatalytic activity and stability for the electro-oxidation of methanol in alkaline methanol fuel cells as compared to that on conventional acid-treated MWCNTs and carbon black indicated this research work will be of great significance and is very promising for portable applications in DMFC in alkaline solution.

Section snippets

Materials

All chemical reagents used in this experiment were of analytical grade. Sulfuric acid, nitric acid, ethanol, methanol, o-aminophenol, PdCl₂, and NaBH₄ were procured commercially and were used as received without further purification. The raw-MWCNTs were purchased from Shenzhen Nanotechnologies Port Co. Ltd. (Shenzhen, China) with the diameters of 40–60 nm, lengths of 5–15 μm, and purity of 98%. Vulcan XC-72 carbon black was purchased from Cabot Corporation.

Acid- and APZ-functionalization of MWCNTs

The acid-treated MWCNTs (denoted as

Mechanism of the Pd/APZ-MWCNTs formation

o-aminophenol was chemically oxidized using nitric acid solution with the help of ultrasonic at 60 °C for 40 min, then we can get a reddish-brown product which was characterized as 2-aminophenoxazin-3-one (APZ) by infrared spectrum. Similar results have been reported in ever before [20], [21]. The synthesis process can be described as the following reaction scheme

APZ is a bifunctional molecule with a benzooxazine-3-heto and an amino functional group. The benzooxazine-3-heto, being highly aromatic

Conclusion

In this paper, we have reported a novel method to prepare Pd nanoparticles supported on MWCNTs. The synthesis process is simple and without use expensive chemicals. Cyclic voltammetry in the alkaline solution showed that Pd/APZ-MWCNTs catalyst has the largest electrochemical active surface area. Study on the electro-oxidation of methanol on the catalysts showed that Pd/APZ-MWCNTs have a much higher specific activity than that of Pd nanoparticles supported on AO-MWCNTs and carbon black. The

Acknowledgements

This work has been supported by the Natural Science Foundation of Guangxi Province (0728043), Guangxi Ministry of Education and Innovation Plan in Graduate Education of Guangxi Province (2009106020703M49).

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Highly dispersed Pd nanoparticles on 2-aminophenoxazin-3-one functionalized MWCNTs surface for methanol electro-oxidation in alkaline media

Abstract

Introduction

Section snippets

Materials

Acid- and APZ-functionalization of MWCNTs

Mechanism of the Pd/APZ-MWCNTs formation

Conclusion

Acknowledgements

Electrochim. Acta

Electrochem. Commun.

Carbon

Electrochem. Commun.

J. Power Sources

J. Power Sources

Electrochem. Commun.

Electrochim. Acta

J. Mol. Catal. A: Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

Talanta

Electrochim. Acta

Int. J. Hydrogen Energy

Int. J. Hydrogen Energy

Electrochem. Commun.

J. Power Sources

Int. J. Hydrogen Energy

Chem. Mater.