Elsevier

Electrochimica Acta

Volume 55, Issue 8, 1 March 2010, Pages 2911-2917
Electrochimica Acta

High efficient electrocatalytic oxidation of formic acid on Pt/polyindoles composite catalysts

https://doi.org/10.1016/j.electacta.2010.01.017Get rights and content

Abstract

Four novel composite catalysts have been developed by the electrodeposition of Pt onto glassy carbon electrode (GCE) modified with polyindoles: polyindole, poly(5-methoxyindole), poly(5-nitroindole) and poly(5-cyanoindole). As-formed composite catalysts are characterized by SEM, XRD and electrochemical analysis. Compared with Pt nanoparticles, respectively, deposited on the bare GCE and on the GCE modified with polypyrrole, the four newly developed composite catalysts exhibit higher catalytic activity towards formic acid electrooxidation by improving selectivity of the reaction via dehydrogenation pathway and thus mostly suppressing the generation of poisonous COads species. The enhanced performance is proposed to come from the synergetic effect between Pt and polyindoles and the increase of electrochemical active surface area (EASA) of Pt on polyindoles.

Introduction

The direct formic acid fuel cell (DFAFC) has attracted great attention because formic acid is non-toxic and low crossover compared to methanol [1]. Pt is commonly used as the electrocatalyst for formic acid oxidation. It has been widely accepted that the mechanism of formic acid oxidation on Pt catalysts involves the dual-pathway mechanism consisting of a direct pathway without COads-like poisoning and an intermediate pathway with COads-like poisoning [2], [3], [4], [5], [6], [7], [8], [9]. Generally, the intermediate pathway occurs readily on Pt catalyst during the formic acid electrooxidation, which significantly reduces its catalytic performance by blocking of active sites. In order to mitigate COads-like poisoning, one perfect way is to redirect the reaction from intermediate pathway to direct pathway. In this context, Pt-based bimetallic catalysts [10], [11], Pt modified with Fe-macrocycle compounds [12], [13], [14], and Pt modified with manganese oxide single crystalline nanorod [15] were reported, which showed an enhanced activity for the direct oxidation of formic acid. However, these approaches involved high cost due to the complicated preparation processes and costly materials. Therefore, it is desirable to develop an easier and cheaper route to fabricate Pt-based composite catalysts which can promote the direct oxidation of formic acid.

Conducting polymers (CPs) have received an increasing attention due to their potential applications in catalysis, sensors, microelectronics, etc. [16], [17], [18]. The activity of catalysts on CPs was greatly enhanced for the oxidation of organic molecules possibly due to their high surface area and/or the synergistic effect between CPs and catalysts [19], [20]. The studies of CPs as host material of Pt nanoparticles were centered on the polyaniline [8], [21], polypyrrole (PPy) [7], polythiophene [22] for the formic acid electrooxidation. However, polyindole and its derivatives (PIns) owning the properties of poly(para-phenylene), polyaniline and PPy together had received a significant share of attention [23], [24], [25]. High quality PIns were prepared from boron trifluoride diethyl ether, such as polyindole (PIn) [26], poly(5-cyanoindole) (PCI) [27], poly(5-methoxyindole) (PMI) [28] and poly(5-nitroindole) (PNI) [29]. They have high conductivity (about 10−2 S cm−1), strong adhesion to the electrode substrate, good electrochemical activity and environmental stability. To the best of our knowledge, up to now, there have been little reports on using PIns as the support for Pt nanoparticles in DFAFC.

In the present work, four composite catalysts have been successfully prepared by the electrodeposition of Pt onto glassy carbon electrode (GCE) modified with PIn, PCI, PMI and PNI, respectively. As-formed composite catalysts were characterized by SEM, XRD and electrochemical method. The electrocatalytic activities of these composite catalysts were examined by cyclic voltammetry and chronoamperometric method in the presence of 0.5 M formic acid and 0.5 M H2SO4.

Section snippets

Materials

Indole, 5-cyanoindole, 5-methoxyindole and 5-nitroindole (Analytical grade, Shandong Pingyuan Hengyuan Chemical Co., Ltd., China) and H2PtCl6 (Shanghai Shiyi Chemicals Reagent Co., Ltd., China) were used as received. Pyrrole (Fluka, 97%) was distilled under a nitrogen atmosphere just prior to use. Boron trifluoride diethyl ether (BFEE, distilled before use) was a product of Sinopharm Chemical Reagent Co., Ltd. Diethyl ether (EE) and formic acid were of analytical-grade purity. Doubly distilled

Electrochemical syntheses of films on GCE

The successive cyclic voltammograms (CVs) of 0.05 M indole and its derivatives in BFEE and its mixed electrolyte on GCE are shown in Fig. 1. As can be seen from the Fig. 1, the CVs of indole and its derivatives show characteristic features as other conducting polymers such as polythiophene and PPy during potentiodynamic syntheses [33], [36]. As the CV scan continues, a polymer film was formed on the working electrode surface. The increase in the redox wave currents implied that the amount of the

Conclusions

In this paper, four novel composite catalysts (Pt/PIn, Pt/PMI, Pt/PNI and Pt/PCI) for the electrooxidation of formic acid have been fabricated by an electrochemical method. It is the first time to find that PIns are active in promoting the electrooxidation of formic acid. Compared with their counterparts Pt/PPy/GCE and Pt/GCE, the four composite catalysts exhibited higher catalytic activity for formic acid electrooxidation; the onset oxidation potential of formic acid was lowered significantly

Acknowledgements

This study was supported by NSFC20933007, 50963002, the key scientific project from Ministry of Education, China (2007-207058), Natural Science Foundation of Jiangxi province (2007GZH1091), Jiangxi Provincial Department of Education (GJJ08369), the Foundation of Jiangsu Province Education Committee (08KJD150020), funded by pre-research project of Soochow University.

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