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A polyoxometalate-deposited Pt/CNT electrocatalyst via chemical synthesis for methanol electrooxidation

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Abstract

Polyoxometalate anion PMo12O403− (POM) is chemically impregnated into a Pt-supported carbon nanotubes (Pt/CNTs) catalyst that is prepared via a colloidal method. The POM-impregnated Pt/CNTs catalyst system (Pt/CNTs-POM) shows at least 50% higher catalytic mass activity with improved stability for the electrooxidation of methanol than Pt/CNTs or POM-impregnated Pt/C (Pt/C-POM) catalyst systems. The enhancement in electrochemical performance of the Pt/CNTs-POM catalyst system can be attributed to the combined beneficial effects of improved electrical conductivity due to the CNTs support, highly dispersed Pt nanoparticles on the CNTs, and increased oxidation power of the polyoxometalate that can assist oxidative removal of reaction intermediates adsorbed on the Pt catalyst surface.

Introduction

The direct methanol fuel cell (DMFC), which is generally fabricated with an acidic polymer electrolyte and Pt-based electrodes, has been increasingly attracting attention as a clean alternative power source for portable electronic devices. It offers the advantages of high-specific energy density and good conversion efficiency of methanol fuel at a relatively low operating temperature [1], [2]. Prior to further implementation, however, technological problems such as low power density due to sluggish oxidation reactions at the Pt-based anode, fuel crossover from negative (anode) to positive (cathode) electrode, and the high price and limited supply of the electrode catalysts have yet to be overcome [1]. To improve electrocatalyst systems in DMFC electrodes with economical use of the Pt content, alternative supports, various bimetallic catalysts and modified preparation methods have been developed. For instance, carbon nanotubes (CNTs) have drawn increasing attention in recent years for their applications as electrocatalyst supports [3], [4], [5], [6], [7] because the CNTs have good electrical properties, chemical stability in acidic electrolyte conditions and high specific surface-area.

Polyoxometalates (POMs) are inorganic clusters with oxometalates that can be readily prepared through the coordination chemistry of a central heteroatom surrounded by addenda ions of metal oxides [8]. Due to their electrochemical redox properties and oxidizing abilities, the POMs have been applied to various kinds of acid-catalyzed oxidation reactions [9], [10]. Also it was demonstrated that Keggin-type PMo12O403− anions in an aqueous solution could effectively assist the electrochemical oxidation of carbon monoxide (CO) with water molecules to carbon dioxide (CO2) over gold catalysts [11], [12], as represented byCO (g) + H2O (l) + PMo12O403− (aq)  CO2 (g) + 2H+ (aq) + PMo12O405− (aq)Note that CO is a byproduct that is generally produced during the electrooxidation of methanol, but it should be catalytically or electrochemically removed from the fuel electrode as CO can seriously poison Pt-based electrocatalysts [5]. As the POM anion has been shown to be readily adsorbed on gold, carbon and mercury electrodes [13], [14], [15], as well as, on the surface of CNTs [16], there was a report to demonstrate that PMo12O403− anions were incorporated into CNT supports and that POM-modified CNT composites could be employed as a good support for methanol electrooxidation, in which Pt or PtRu was electrochemically deposited on POM-modified CNTs that were mounted on a pyrolytic graphite (PG) electrode [16].

A colloidal method for nanoparticle formation has been widely employed to prepare Pt-based fuel cell catalysts with narrow particle size distribution and high dispersion [17], [18], [19]. Thus, POM modification of Pt/CNT catalysts made from this colloidal preparation would provide a further advantage for DMFC electrode catalysts by utilizing the oxidizing power of the POM. POMs are easily decomposed, however, by hydrolysis in aqueous basic solutions [20] that is normally employed in the colloidal synthesis process with reducing agents of polyols [17].

We report here that POM-impregnated Pt/CNT (hereafter denoted as Pt/CNT-POM) can be obtained by simple chemical synthesis methods, in which highly dispersed Pt nanoparticles are first prepared and deposited on to the surface of CNTs via a polyol-based colloid method, and POMs can be subsequently deposited on to the Pt/CNTs in an aqueous solution. Their catalytic properties in terms of mass activity and stability for methanol electrooxidation are then compared with Pt/CNTs, Pt/C-POM or PtRu/C to investigate the effects of CNTs and POM over the chemically prepared Pt/CNT-POM catalyst systems.

Section snippets

Experiment

The CNTs (Iljin Nanotech Co., CM-95, BET surface area = 200 m2 g−1) were ultrasonicated and stirred in an acid solution of HNO3 and HCl with a volume ratio of 1:3 for 12 h to purify and activate the CNT supports. The CNTs were then filtered and washed with distilled water (18.2  cm) [21] and dried by vacuum freeze-drying to prevent aggregation. Next, the CNT-supported Pt catalysts (20 wt.% in metal loading) were prepared through a colloidal method with ethylene glycol, in which the latter served as a

Results and discussion

Fig. 1 shows the cyclic voltammograms (CVs) of the synthesized Pt/CNT and commercial Pt/C catalysts in an electrolyte solution of 0.5 M H2SO4 + 2 M CH3OH. There are two irreversible current peaks during the electrooxidation of methanol that are typically attributed on the forward scan peak at around 0.7 V to methanol electrooxidation and on the backward peak at ca. 0.4 V, to the Faradaic oxidation reaction on the Pt of residual intermediate species such as CH2OH, CH2O, HCOOH and CO [23], [24]. Both

Conclusions

The polyoxometalate anion PMo12O403− (POM) was chemically impregnated into Pt-supported carbon nanotube (Pt/CNT) catalysts. The Pt/CNT-POM catalysts have superior Pt-based mass activities with improved stability in the electrooxidation of methanol, as compared with Pt/CNTs, Pt/C, and PtRu/C. The enhancements in activity and stability over Pt/CNT-POM catalyst systems have been attributed to several combined beneficial effects, such as: (i) improved electrical conductivity of the CNTs; (ii)

Acknowledgements

This work was supported by New & Renewable Energy R&D Program (2005-N-FC03-P-01-0-000) under the Korea Ministry of Commerce, Industry and Energy (MOCIE) and the Korea Research Foundation Grant of the Korean Government (MOEHRD) (KRF-2005-205-D00023).

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