Elsevier

Chemical Physics Letters

Volume 379, Issues 1–2, 19 September 2003, Pages 99-104
Chemical Physics Letters

Composite electrodes made of Pt nanoparticles deposited on carbon nanotubes grown on fuel cell backings

https://doi.org/10.1016/j.cplett.2003.08.021Get rights and content

Abstract

Multiwalled carbon nanotubes (MWCNTs), with typical lengths of 20 μm and diameters of 40 nm, have been grown directly on carbon paper backing. A sulfonic acid–silicate intermediate was used to deposit Pt particles on the MWCNTs in order to obtain an electrode that could be used in electrocatalysis. The electrical path between the Pt nanoparticles (1.2 ± 0.3 nm in size) and the carbon paper, through the MWCNTs, was demonstrated by cyclic voltammetry. The Pt surface density of a typical MWCNT composite electrode is estimated to be about 25% of that of an ELAT electrode from E-TEK, containing 0.4 mg Pt/cm2.

Introduction

Carbon nanotubes are of significant interest, due to their unique structure, properties and potential applications [1], [2], [3]. If the attention today is mainly focused on the synthesis, purification, functionalization and possible applications of single walled carbon nanotubes (SWCNTs), multiwalled carbon nanotubes (MWCNTs) are certainly closer to commercial applications. For instance, the use of MWCNTs in composite materials [4], or as electrical field emitters [5], is well established. Our interest in MWCNTs is to use them as electrocatalyst supports in polymer electrolyte membrane (PEM) and in direct methanol fuel cells. Indeed, the use of carbon nanostructures as metal supports in electrochemistry and fuel cells has been reported by several authors [6], [7], [8], [9], [10], [11]. However, in these reports, the catalysts were deposited onto loose nanotubes, or nanohorns, just as on carbon black. Our strategy for the electrode fabrication in this work is different.

We recently developed a unique CVD procedure permitting us to grow MWCNTs on the carbon fibers of a fuel cell backing. In the present work, we will show that it is possible to deposit Pt particles on these MWCNTs and, therefore, to obtain a composite electrode for which an electrical path exists between the Pt catalyst and the carbon backing. The deposition of Pt on MWCNTs has been chosen because Pt and its alloys are currently the most extensively used catalysts in PEM and direct methanol fuel cells [12], [13], [14].

Carbon nanotubes are hydrophobic materials, rendering difficult the adhesion of metal deposits. It has been reported that carbon nanotubes cannot be wet by liquids with surface tension higher than 100–200 mN/m [15], indicating that most metals would not adhere to them. In order to improve metal deposition onto nanotubes, two main approaches were previously developed: (i) surface modification and (ii) sensitization activation. The former is associated with the oxidation of the nanotube surface, in order to create functional groups and increase metal nucleation [16], [17]. The latter involves the generation of small nuclei (e.g., of Pd–Sn) to further promote metal deposits on carbon nanotubes [18], [19].

In our previous studies on the growth of MWCNTs [20], [21], we successfully used a sulfonic acid–silicate intermediate to disperse Co–Ni particles, used in the catalytic decomposition of ethylene, to promote the growth of nanotubes on the carbon backing (see Fig. 1). In this work, we exploit the same idea to deposit Pt particles onto MWCNTs grown on carbon paper. The present work demonstrates that it is possible to obtain very small Pt particles deposited onto MWCNTs and that these metal particles are in electrical contact, through the MWCNT, with the carbon backing, enabling the composite structure to be used as an electrode.

Section snippets

Experimental

MWCNTs were synthesized by heating Co–Ni particles deposited on the fibers of a carbon paper in a specifically designed CVD reactor [22]. This reactor takes advantage of the fact that the carbon fuel cell backing (E-TEK, Division of DeNora with 81% porosity) is the most resistive part of the electrical circuit. It can therefore be heated by Joule effect, up to 800 °C, a temperature high enough to decompose ethylene and grow nanotubes. Prior to Co–Ni deposition, the carbon backings were

Results and discussion

A typical SEM micrograph of MWCNTs, grown on the fibers of a carbon paper, is shown in Fig. 2a. Fig. 2b displays a TEM close-up of the same MWCNTs. A high density of tubes, firmly anchored on the carbon paper, resulted from the methanol pretreatment of the carbon paper before adsorbtion of the Co–Ni sulfonate–silicate. The latter is used as precursor of the Co–Ni particles, which are the catalysts used for the growth of the nanotubes [20], [21]. These tubes are about 20 μm in length and are

Conclusions

It has been demonstrated that highly dispersed, and very small (1.2 ± 0.3 nm), Pt particles have been successfully deposited on the surface of MWCNTs grown on the fibers of carbon paper used as fuel cell backing. Cyclic voltammetry strongly suggests that there is an electrical contact through MWCNTs between the Pt particles and the carbon backing, enabling the use of these composite structures as electrodes. The Pt surface in contact with the electrolyte is estimated to be about 25% of that of a

Acknowledgments

This work was supported by Le Ministère de la Défense Nationale et les Ministères des Ressources Naturelles du Canada et du Québec.

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