Fabrication of bimetallic Pt–M (M = Fe, Co, and Ni) nanoparticle/carbon nanotube electrocatalysts for direct methanol fuel cells
Introduction
Fuel cells have been hailed as an important power source for the future because of their high energy conversion efficiency and low environmental pollution [1], [2], [3], [4], [5], [6]. The conversion of chemical energy into electricity in direct methanol fuel cells (DMFCs) requires the development of better catalysts to improve the cell performance. It is generally recognized that pure Pt electrocatalysts are prone to poisoning by CO since the CO molecules can chemically adsorb onto the Pt surface and block the active sites [7], [8], [9]. As a result, Pt catalysts rapidly deactivate owing to the formation of Pt–CO species in the electrooxidation of methanol. To date, the development of bimetallic catalysts usually consists of a primary metal that has a high performance in catalytic activity and a secondary metal that can enhance the catalytic activity or prevent poisoning problems. It has been shown that bimetallic PtRu catalysts enable the reduction of CO poisoning [8], [9], [10], [11], [12], [13], [14], [15], [16]. The role of ruthenium is to dissociate water to produce adsorbed OH species, which react with CO adsorbed on the Pt surface to generate CO2. More recently, an attempt to increase the activity by alloying Pt with a second metal such as Sn [17], Co [18], [19], [20] and other metals [21], [22], [23] has been reported. The appearance of the second metals exhibited a remarkable improvement in CO tolerance and electrochemical activity. Moreover, the presence of a second element more abundant could contribute to decreasing of costs associated with Pt [18].
To improve the electrochemical activity, a common approach has been used to uniformly deposit bimetallic catalysts onto a carbon support. Commercial anode catalysts in DMFCs are frequently PtRu nanoalloys coated on carbon black in the form of well dispersion of Pt50Ru50 particles [8]. There has been increasing interest in multiwalled carbon nanotubes (CNTs) as heterogeneous catalyst supports [8], [9], [10], [11], [12], [13], [14], [15], [24], owing to their high porosity and high electrical conductivity. However, the role of deposition of bimetallic catalysts on CNT supports in the improvement of electrochemical activity in methanol oxidation has not yet been clearly elucidated. Our pervious study has shown an efficient way to deposit electrocatalysts on the sidewalls of CNTs [9], [25], [10]. Proper dispersion of the catalysts on CNTs would enhance the reaction kinetics and activity for methanol oxidation, since CNTs have a unique one-dimensional structure. This work aims to demonstrate the electrochemical characterization of Pt–M/CNTs electrocatalysts, including the onset potential, electrochemical activity, and cycle stability in methanol oxidation. These results would shed some light on the replacement of novel metal catalysts by using transition metals, and on how the introduction of transition metals in bimetallic catalysts enhances the electrochemical performance of methanol oxidation.
Section snippets
Synthesis of Pt–M/CNT composites
The procedure for growing different types of Pt–M (M = Fe, Co, and Ni)/CNT composites can be described as follows. In this study, multiwalled CNTs (purity: >99%; outer diameter: 30–50 nm; length: 5–10 μm) were prepared by a catalytic chemical vapor deposition technique, using ethylene and Ni particle as the carbon precursor and catalyst, respectively. A chemical-wet oxidation in nitric acid enabled the implantation of surface oxides such as carboxylic (–COOH), carbonyl (–CO), and hydroxyl (–C–OH)
Morphology and crystalline structure of Pt–M/CNT composites
Typical SEM images for each pair of the bimetallic nanoparticles deposited on oxidized CNTs are shown in Fig. 1(a)–(d). The metal nanoparticles are homogeneously dispersed on the surface of CNT support. In comparison, the average sizes of bimetallic Pt–M particles are found to be very close to that of Pt, indicating a good incorporation of transition metals into the Pt catalyst. An analysis of energy-dispersive spectroscopy (EDS) shows that the element compositions of the bimetallic catalysts
Conclusions
In this paper we have demonstrated an efficient fabrication of bimetallic Pt–M (M = Fe, Co, and Ni) catalysts in the electrooxidation of methanol, using oxidized CNTs as the catalyst support, and ethylene glycol and hydrogen as the reduction agents. Under our experimental conditions, three catalysts with a similar Pt:M atomic ratio, Pt–Fe (75:25), Pt–Co (75:25), and Pt–Ni (72:28), were prepared for the investigation of methanol oxidation. SEM and TEM analyses showed that highly dispersed
Acknowledgements
The authors gratefully acknowledge financial support from the Ministry of Education (MOE) and the National Science Council (NSC) in Taiwan, through Project NSC 96-2221-E-155-055-MY2.
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