Novel ionic liquid supported synthesis of platinum-based electrocatalysts on multiwalled carbon nanotubes

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Abstract

Novel ionic liquids (ILs) were used to prepare carbon nanotube (CNT) supported platinum catalysts for the first time. The CNTs were pretreated in acid to get functional groups to promote the adherence of Pt nanoparticles. Ionic liquids added to the reaction system are suggested to contribute to the formation of homogeneous small Pt nanoparticles and to suppress the agglomeration of CNTs. The Pt/CNT electrocatalyst has a fairly homogeneous dispersion of Pt particles with small particle sizes. The electrochemical performance of the Pt/CNT electrocatalysts for PEM fuel cells was studied by cyclic voltammetry measurements. The results show that the ionic liquid has a significant impact on the surface of the as-prepared CNTs and that there is a strong interaction between ionic liquids and CNTs.

Introduction

Proton exchange membrane (PEM) fuel cells have recently attracted great interest for their potential as clean and mobile power sources. Noble metal, such as Pt, Ru or PtRu alloys, supported on high surface area carbons are typically used as electrocatalysts for PEM fuel cells. As noble metals are expensive, the synthesis of these metal particles as electrocatalytic materials with uniform size and good dispersion on the carbon supports has thus become essential.

Many investigators have demonstrated that carbon nanotubes (CNTs) can be used as catalyst supports [1], [2], [3], [4], [5]. Quinn et al. [6] recently described the electrodeposition of noble metal nanoparticles on carbon nanotubes under potential control . The metal particles can decorate the external walls of the CNTs resulting in hybrid materials with high electrocatalytic activity. Since the CNTs are chemically inert, it is essential to activate their surfaces. Chemical treatments are common methods to generate functional groups on CNTs [7], [8], [9], [10]. Generation of functional groups on the surface of CNTs can improve the reactivity and provides an avenue for further chemical modification of the carbon nanotubes, such as by ion adsorption, metal deposition and grafting reactions. It has been found that the chemical modification of the surface of CNTs by using HNO3 or a H2SO4–HNO3 mixture can improve the metal dispersion. However, it is difficult to decorate metal nanoparticles on the surface of CNTs with uniform size and good dispersion. One of the reasons is that the metal nanoparticles are spontaneously formed at the defect sites on the surface of CNTs. Moreover, carbon nanotubes tend to agglomerate without any pretreatments.

In this paper, Pt nanoparticles decorated CNTs have been successfully synthesized by using novel ionic liquid modified CNTs. ILs are liquids at room temperature and represent a new series of nonaqueous but polar solvents, which are nonvolatile, exhibit high conductivities and possess no vapor pressure [11], [12], [13], [14], [15]. The monodispersed Pt nanoparticles on modified CNTs were prepared by a microwave-heated polyol process. The microwave-heated polyol process has been demonstrated to be an effective way to prepare polymer stabilized metal nanoparticles, such as Pt, Ag, Au and Pd [16], [17], [18], [19]. Ethylene glycol is commonly used as the solvent due to its high dielectric constant (41.4 at 298 K) and dielectric loss, and can be rapidly heated by microwave radiation. Fast heating rates can accelerate the formation of the Pt nanoparticles, and the uniform microwave irradiation provides more homogeneous circumstances for the nucleation and growth of the Pt nanoparticles.

Section snippets

Microwave heated synthesis of Pt/CNTs

The multiwalled CNTs were prepared by catalytic CVD using nano-sized cobalt as the catalyst [20]. The CNTs were purified in concentrated HNO3 at 393 K for 2 h. Pt/CNTs catalysts were synthesized from a microwave heated H2PtCl6 ethylene glycol (EG) solution with ionic liquids. 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI · TFSI) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMI · TFSI) were synthesized as described previously [21]. The schematic structure of

Results and discussion

Since the CNTs prepared by the catalytic CVD method are accompanied by many other types of carbon particles, it is essential to use a strong oxidant to purify the raw carbon nanotubes. The CNTs used in this work were pretreated with HNO3 at 393 K, which could result in many functional groups, such as carboxyl, hydroxyl and carbonyl groups on the surface of the CNTs. By a coordination reaction or an in-exchange reaction, platinum ions interact with and attach to these functional groups on the

Conclusions

Good quality electrocatalysts often depend on their synthesis technique. Highly uniform dispersion of the metal particles is the basic requirement for carbon supported Pt catalysts. Compared to the previous technique for the synthesis of Pt-based carbon nanotube electrocatalysts, novel ionic liquids are likely to contribute to the formation of small Pt particles and suppress the agglomeration and tangling of the as-prepared CNTs. The Pt/CNTs prepared in this work seem to be promising

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