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


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][4][5][6][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][13][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 [22,23].In addition, ILs are also useful solvents and stabilizers for the preparation of various metal NPs, mainly because of having high intrinsic charges that allow them to stabilize metal NPs through electrostatic attractions [24][25][26].Having such excellent properties to stabilize CNTs and Pt NPs [27], it is worthy to prepare Pt NPs/CNT nanohybrids in the presence of ILs.To prepare Pt NPs/CNT nanohybrids, 1-butyl-

Four ILs, including [BMIM][PF 6 ], [BMIM][Cl], [BMIM][Br], and [BMIM]
[I], were used separately to prepare Pt/ILs/CNT nanohybrids.In a typical procedure, CNT (0.4 mg) and [BMIM][PF 6 ] (400 μL, 4%) were mixed with DMF to obtain a final volume of 10.0 mL.We found that sonicating the mixture for longer time (>5min) did not increase the dispersing ability of CNTs.To avoid sonication-induced damage to the CNTs, 5 min sonication time was found to be optimal for our study.The sonication power and frequency were kept low and cold water with a surfactant was used in the sonicator bath to avoid heat-induced damage to the CNTs.The solution was thus subjected to sonication for 5 min and then excess DMF was removed through centrifugation at a relative centrifuge force (RCF) of 7370 g for 10 min.The black precipitates were added to a mixture of ethylene glycol (5.7 mL) and ultrapure water (2.3 mL).The mixture was subjected to sonication for 5 min and then K 2 PtCl 6 (2.0 mL, 0.6 mM) was added.The reaction mixture was kept at 115 °C for 1 h to form Pt/ILs/ CNT nanohybrids.The as-prepared Pt/ILs/CNT nanohybrids were finally purified through sonication for 5 min and four cycles of centrifugation (RCF 7370 g, 10 min)/wash (ultrapure water, 4 Â 10 mL).

Characterization
Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of as-prepared Pt/ILs/CNT nanohybrids were recorded using JEOL JSM-1230 (Hitachi, Tokyo, Japan) and FEI Tecnai-G2-F20 (GCEMarket, NJ, USA) systems operating at 200 kV, respectively.Energy-dispersive X-ray spectrometer (EDS) equipped with a Hitachi S-4800 field emission scanning electron microanalyzer (Hitachi, Tokyo, Japan) was applied to determine the composition of the asprepared Pt/ILs/CNT nanohybrids.The X-ray diffraction (XRD) patterns of the as-prepared Pt/ILs/CNT nanohybrids were recorded using a PANalytical X'Pert PRO diffractometer (Almelo, Netherlands) with Cu Kα radiation (λ = 0.15418 nm).X-ray photoelectron spectroscopy (XPS) measurement was conducted using a VG ESCA210 electron spectroscope from VG Scientific (West Sussex, UK) to determine Pt oxidation state in the as-prepared Pt/ILs/CNT nanohybrids.The binding energy (BE) of carbon was used as an internal reference to eliminate the charging effect.The samples for XPS measurements were prepared by depositing aliquots of the as-prepared Pt/ILs/ CNT nanohybrids onto Si substrates and then the solvents were evaporated at ambient temperature (25°C) and pressure.An Elan 6000 inductively coupled plasma mass spectrometer (ICP-MS) from Perkin-Elmer (Wellesley, MA, USA) was employed to determine Pt contents in the as-prepared Pt/ILs/CNT nanohybrids.Prior to ICP-MS measurements, the as-prepared Pt/ILs/CNT nanohybrids were dissolved in 2% HNO 3 .

Fabrication of electrodes and electrocatalytic analysis
Aliquots (5 mL) of the as-synthesized Pt/ILs/CNT nanohybrids solutions were dropped separately onto clean screen-printed carbon electrodes (diameter: 3 mm).After the electrodes were air-dried for 1 h at ambient temperature, Nafion solution (0.5%, 1 mL) was placed onto each of the electrodes.Threeelectrode electrochemical cells were fabricated using the modified electrode as a working electrode, a Pt wire as an auxiliary electrode, and an Ag/AgCl electrode as a reference electrode.The electrocatalytic activities of the as-synthesized Pt/ILs/CNT nanohybrids were measured using a CH Instruments (CHI) 760D electrochemical workstation (Austin, TX, USA).Cyclic voltammetry (CV) measurements in 0.5 M KOH with or without containing 0.5 M methanol were conducted over the potential range from 1.0 to 0.2 V at a scan rate of 100 mV s 1 .As a control, commercial Pt/C NPs and assynthesized Pt/CNT nanohybrids prepared without ILs were used to prepare working electrodes under the same conditions.All electrochemical data were recorded over 50 reproducible cycles.The chronoamperometric measurements for durability tests were conducted in 0.5 M KOH containing 0.5 M methanol at a fixed potential of 0.1 V for 20,000 s.

RESULTS AND DISCUSSION Formation and characterization of Pt/ILs/CNT nanohybrids
CNTs were stabilized in the aqueous solutions through the π-π interaction of CNTs with BMIM group of the ILs.PtCl 6 2 ions were then adsorbed on the cationic surfaces of the CNTs through electrostatic interactions [28].The adsorbed PtCl 6 2 ions were reduced by ethylene glycol (reducing agent) to form Pt NPs on the surfaces of CNTs.Having a weaker reducing strength than NaBH 4 and a capability for rapid and homogenous in-situ generation of reducing species, ethylene glycol allowed better control of the particle growth, leading to the formation of a fairly uniform Pt NPs on the surfaces of CNTs [29,30].The Pt NPs are formed through reactions (1) and (2) [31]:  200), respectively, in reference to JCPDS card no.87-0646 [33].The peaks at 25.9°, 42.7°, and 54.2°c orrespond to the (002), (100), and (004) planes of graphitized CNTs, in reference to JCPDS card No. 75-1621 [33].The results reveal that the as-synthesized nanohybrids contain pure crystalline Pt and CNTs [25].The XPS spectra depicted in Figure 2A and 2B show a doublet of Pt 4f peaks at 71.6 eV (4f 7/2 ) and 74.8 eV (4f 5/2 ), respectively, which correspond to the metallic Pt [34].After deconvolution, six peaks were identified as shown in the dotted curves [29].The 4f 7/2 peak at 72.4 eV with a 4f 5/2 component at 75.6 eV is attributed to the Pt (II) chemical states of PtO or Pt(OH) 2 [34].While the 4f 7/2 peak at 73.4 eV with a 4f 5/2 component at 76.8 eV is attributed to the +4 oxidation state of Pt. Figure 2C and 2D shows the C 1s spectra, which exhibits a main peak at 284.7 eV that is assigned for the C 1s of the sp 2 -hybryidized graphitic carbon [35].A peak at 285.3 eV is assigned to sp 3 -hybridized carbon atoms that are found to be same as that of diamondlike carbon [35].We note that these two peaks are observed in amorphous carbons, and their relative intensity correlates with the degree of graphitization [36].Peaks with higher binding energies located at 286.0, 287.5, and 289.4 eV are assigned to CO (e.g., alcohol, ether), >C=O (ketone, aldehyde), and COO (carboxylic acid, ester) functional groups, respectively [33,35].The peak at 286.7 eV is assigned to CN group of BMIM [37].We note that the electronegative oxygen atoms induce the formation of more positive charge on a carbon atom.The bonding configurations (Figure 2E and 2F) of the nitrogen atoms in the Pt/[BMIM][PF 6 ]/CNT and Pt/ [BMIM][Cl]/CNT nanohybrids were also fitted.The peak at 399.8 eV in the N 1s spectrum corresponds to pyrrole-like nitrogen.When carbon atoms in the CNT surface are substituted with nitrogen atoms in the form of "graphitic" nitrogen, the corresponding peak is located at 401.5 eV [2,[38][39][40].The functionalization of CNTs with nitrogencontaining materials is beneficial to enhance the dispersion of Pt NPs on the CNTs due to a strong coordinative interaction between nitrogen atoms and Pt NPs [6,12,13].

Electrocatalytic activities of Pt/ILs/CNT nanohybrid-modified electrodes
Prior to testing the electroactivity of the as-prepared Pt/ILs/ CNT nanohybrids for MOR, we estimated their electroactive surface areas (EASA, m 2 g 1 ) by conducting CV measurement [41].The CVs of Pt/ILs/CNT nanohybrid-modified electrodes at a scan rate of 100 mV s 1 are displayed in Figure 3A.The EASA of each electrode was calculated using equation (3): where [Pt] represents the Pt loading (mg cm 2 ) in the electrode.EASA was calculated from integrating the charges associated with the hydrogen adsorption-desorption peaks (Q, mC cm 2 ), assuming 210 μC cm 2 needed for a monolayer of H as-atoms in Figure 3A  The results reveal that the counter ions of ILs affected the adsorbed amounts of Pt NPs, mainly because the species of anions, cations, and the length of the lateral alkyl groups on the heterocyclic rings affect the physicochemical properties of ILs [27].Since PF 6 relative to Cl has higher steric effect, it is more difficult for PtCl nanohybrid-modified electrode in alkaline solution (0.5 M KOH) is higher than that (242.3A g 1 , and 155.7 A g 1 ) for the PtRu/CNTs-PIL and Pt/CNTs-PIL modified electrodes [44], respectively in the acidic solution (0.5 M H 2 SO 4 ), showing that higher EASA and alkaline conditions are essential to provide higher mass activity for MOR.It has also been reported that the polarization characteristics of MOR at the unsupported Pt black in alkaline solution is one order higher than that in the acidic solution [45].

CONCLUSIONS
We have demonstrated a facile IL-assisted method for one-pot preparation of Pt/IL/CNTs nanohybrids, with high mass activity and long durability.Among four tested ILs, [BMIM] [Cl] allowed preparation of the Pt/IL/CNTs that provided the highest mass activity.Our result reveals that the counter ions play a significant role in determining the amounts of Pt NPs on the CNT support.In the presence of ILs, small sizes of Pt NPs on the CNT support were formed, leading to greater mass activity than other reported electrodes.Having advantages of good stability, excellent electrocatalytic activities, and cost
3-methylimidazolium (BMIM)-based ILs with various counter ions were tested, including [BMIM][PF 6 ], [BMIM][Cl], [BMIM][Br], and [BMIM][I].The BMIM based ILs were adsorbed onto the surfaces of CNTs to provide great amount of surface functional groups on their surfaces to stabilize Pt 4+ ions that were then reduced to form well distributed Pt NPs by ethylene glycol.