Active and stable platinum/ionic liquid/carbon nanotube electrocatalysts for oxidation of methanol

Platinum (Pt) nanoparticles (NPs) on carbon nanotubes (CNTs) have been prepared from PtCl6 2 ions through a facile ionic liquid (IL)-assisted method and used for methanol oxidation. 1-Butyl-3-methylimidazolium (BMIM) with four different counter ions (PF6 , Cl , Br , and I ) has been tested for the preparation of Pt/IL/CNT nanohybrids, showing the counter ions of ILs play an important role in the formation of small sizes of Pt NPs. Only [BMIM][PF6] and [BMIM][Cl] allow reproducible preparation of Pt/IL/CNT nanohybrids. The electroactive surface areas of Pt/[BMIM][PF6]/CNT, Pt/ [BMIM][Cl]/CNT, Pt/CNT, and commercial Pt/C electrodes are 62.8, 101.5, 78.3, and 87.4 m g , respectively. The Pt/ [BMIM][Cl]/CNT nanohybrid-modified electrodes provide higher catalytic activity (251.0 A g ) at a negative onset potential of 0.60 V than commercial Pt/C-modified ones do (133.5 A g ) at 0.46 V. The Pt/[BMIM][Cl]/CNT electrode provides the highest ratio (4.52) of forward/reverse oxidation current peak, revealing a little accumulation of carbonaceous residues. INTRODUCTION Platinum (Pt) nanomaterials with various morphologies, large surface areas, and high surface energies have been widely used as catalysts for enhanced methanol oxidation reaction (MOR) in direct methanol fuel cells (DMFCs) [1, 2]. In order to enhance the efficiency of DMFCs and to accelerate the electron transfer rate, carbon nanotubes (CNTs) with high specific surface areas, electrical conductivities, and chemical stability have been used as supports for the preparation of Pt nanoparticles (NPs)/CNT nanohybrids [3–7]. However, pristine CNTs have insufficient binding sites to anchor Pt ions (precursors) and NPs, which lead to poor dispersion, aggregation of NPs [8], inefficient catalytic activity, poor reproducibility, and poor durability of Pt NPs/CNT nanohybrids. In order to produce more efficient Pt NPs/CNT nanohybrids, functional CNTs with greater binding sites and surface anchoring groups are usually used [9]. Acid-oxidation is a common approach to produce CNTs with greater binding sites (CO, COH, and COOH groups) that can anchor great amounts of Pt ions and Pt NPs [10, 11]. However, this method typically leads to uneven distribution of surface functional groups as well as severe structural damage to CNTs. Strong acids used to treat CNTs at elevated temperature are a concern. Alternatively, electrochemical oxidation of CNTs at their defect sites is used to produce quinonyl, carboxyl, or hydroxyl groups on their surfaces [12–14]. Chemical modification of CNTs with water-soluble polymers, quaternary ammonium salts, surfactants, and polyoxoanions has become popular for preparation of functional CNTs, mainly because it is easy to purify and disperse the functional CNTs in aqueous solution [15]. However changes in the aromatic structures of CNTs sometimes occurs [16]. With various degrees of π-conjugates (C=C) on their surfaces, aromatic compounds such as pyrene have been conjugated with CNTs through π–π stacking. Although modification of CNTs can be achieved without altering their structure under mild reaction conditions [17], weak π–π stacking force between molecules and CNTs might cause reproducibility and durability problems. It has been shown that Pt NPs tend to get deposited on the defect sites and boundaries of the CNTs, which allows the as-deposited Pt NPs to be surrounded with more oxygen containing functional groups that favors the facile electron and proton transport during the MOR [18]. The inner and outer surfaces of CNTs and grooves at the junction of adjacent CNTs act as chemisorption sites for the Pt NPs [19]. The other factors that affected the distribution and deposition of Pt NPs on the surfaces of CNTs are interaction between the Pt ions and the functional groups, properties of CNTs, and the functional group densities on the outer walls of CNTs [20]. Ionic liquids (ILs) have been found useful for the modification of CNTs [1, 21] as they have excellent properties of good chemical and thermal stability, negligible vapor pressure, good electrical conductivity, and a wide electrochemical window SOR-CHEM