Effects of morphological characteristics of Pt nanoparticles supported on poly(acrylic acid)-wrapped multiwalled carbon
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The catalytic activity of Pt nanoparticles (NPs) significantly influences the electrochemical performance of direct methanol fuel cells. Information about the factors that influence the electrochemical activity of the catalyst themselves is scarce; hence, guidelines for the preparation of Pt NPs that yields the best performances are lacking. With consideration for this situation, we systematically investigated the relationship(s) between the characteristics of Pt NPs and their electrochemical performance. The general characteristics of Pt NPs, such as the average size, loading density, and dispersion status on the support, were varied in the presence of poly(acrylic acid)-wrapped multiwalled carbon nanotubes by controlling the preparation conditions, including the pH of the aqueous solution, the reaction temperature, and the reaction time. The enhanced catalytic activity is attributable to higher degree of dispersion, specific surface area, and electrochemically active surface area of Pt NPs. The optimized catalyst exhibits a ;165% higher catalytic activity toward methanol oxidation than the commercial E-TEK. I. INTRODUCTION
Fuel cells are among the most promising power supply systems for meeting ecofriendly energy demands in a practical manner.1 Direct methanol fuel cells (DMFCs) have been intensively studied for use in portable appliances with the expectation of achieving high-efficiency energy conversion with a lower weight that is achievable using other fuel cell systems.2 However, poor methanol oxidation catalysis at the anodes and the need for expensive platinum catalysts have hampered the widespread use of DMFCs.1–4 Methanol oxidation catalysis is sensitive to a variety of factors, such as the average size, loading amount, and dispersion status of the catalyst particles. To control these factors and enhance the catalytic activity, Pt nanoparticles (NPs) are immobilized on supports, including porous substrates, such as activated carbon,4–8 or nanocarbon, such as carbon black,9,10 carbon nanofibers,11,12 or carbon nanotubes.13–18 Multiwalled carbon nanotubes (denoted MWCNTs) have been widely studied as one of the most effective supporting materials for anchoring Pt NPs due to their excellent electronic conductivity, high accessible surface area, and chemical stability.13 To effectively load Pt NPs onto MWCNTs, the MWCNTs must be covalently or a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.156 J. Mater. Res., Vol. 27, No. 15, Aug 14, 2012
noncovalently modified to create anchoring sites and to enhance the dispersibility of them in a solution.13–24 Covalent surface modification may be achieved by introducing hydroxyl, carboxylic, or thiol groups onto the surfaces of the MWCNTs.13–23 This approach typically concurrently sacrifices the intrinsic properties of the MWCNTs, including the electronic conductivity and the chemical stability, leading to decrease in the catalytic activity of the Pt/MWCNTs and, therefore, the overall DMFC performance. To retain the optimal intrinsic
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