Tailoring carbon nanotubes surface with maleic anhydride for highly dispersed PtRu nanoparticles and their electrocatalytic oxidation of methanol

Abstract

Taking the direct Friedel–Crafts reaction between pristine carbon nanotubes (CNT) and maleic anhydride, we have developed a facile strategy for the synthesis of carboxylated-carbon nanotubes (CNT-C). The covalently grafted carboxyl groups on the CNT-C surface not only significantly improves its hydrophilicity, but also effectively anchors and stabilizes the PtRu nanoparticles. Transmission electron microscopy images reveal that PtRu nanoparticles with an average size of 3.3 nm are uniformly dispersed on the CNT-C surface. Electrochemical results demonstrate the PtRu/CNT-C nanohybrids obtained have a higher electrochemical surface area, electrocatalytic activity and better stability towards methanol oxidation when compared to PtRu nanoparticles supported on acid oxidized-CNT. This provides a facile approach to synthesize noble metal nanoparticles/CNT electrocatalysts for high performance energy conversion devices in the future.

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1. Introduction

Since carbon nanotubes (CNT) were discovered by Iijima in 1991,¹ they have been extensively studied and show great potential applications in a wide variety of areas, such as fuel cells,^2–9 heterogeneous catalysis,¹⁰ and chemo/biosensors.¹¹ Especially, their large specific surface area, high electric conductivity, outstanding chemical and electrochemical stability make CNT an ideal supporting material for noble metal nanoparticles, which have shown great promise in advanced sensors and fuel cells,^12–23 including direct methanol fuel cells (DMFCs). It is well known that Pt is the most popular anode catalyst used towards the oxidation of methanol. However, it is often easily poisoned by the CO intermediate product, which gradually decreases its catalytic activity.^3,5,8 To improve the electrocatalytic activity and stability, various multicomponent catalysts based on Pt have been investigated, such as PtCo,²⁴ PtSn,²⁵ PtRu,^3,5,15 PtCu,²⁶ and Pt–MnO₂.²⁷ Extensive efforts have still paid to further improvements in the electrocatalytic activity of Pt catalysts for their practical applications in DMFCs. Because the properties of the electrocatalyst are highly dependent on their dispersity and particle size,^28–31 highly dispersed metal nanoparticles with small particle size on CNT are desired to enhance the performance of fuel cells with low noble metal loadings in engineering applications. Unfortunately, pristine CNT have insufficient accessible specific surface areas and binding sites for anchoring noble metal nanoparticles owing to their poor dispersibility in solvents and insert graphitic surface.^3,14,32–34 On the other hand, noble metal nanoparticles are kinetically unstable for agglomeration to the bulk metal, which further aggravates the difficulty of uniformly depositing noble metal nanoparticles on pristine CNT.^28,30,35,36

To overcome the above problems, considerable research efforts have been devoted to attach uniform noble metal nanoparticles onto CNT. One of the widely used strategies is acid oxidation treatment of CNT to graft carboxyl groups as binding sites for anchoring noble metal nanoparticles.^28,30 However, because of its limited and uneven distribution of functional groups, noble metal nanoparticles on the acid oxidized-CNT (AO-CNT) generally have large particle size, poor dispersion and agglomeration, which is inferior for gaining high catalytic performance. Meanwhile, non-covalent functionalization methods including wrapping polymers, such as poly(vinylpyrrolidone),³² poly(allylamine hydrochloride),¹⁷ and poly(ethyleneimine)³⁷ onto CNT or modifying CNT with 1-aminopyrene by π–π stacking,³⁸ have attracted particular attention because it can facilely decorate uniform noble metal nanoparticles on the CNT surface without structural damage to the CNT. However, in most cases, these template agents such as polymer, surfactant and macromolecule can strongly bind to the surface of the resulting noble metal nanoparticles and lead to surface poisoning during the electrocatalytic reaction, which is disadvantageous for enhancing electrocatalytic performance of the nanohybrids. The elimination of template agents from nanohybrids is difficult to perform while still keeping the noble metal nanoparticles size and dispersion. Therefore, it is necessary to develop an effective functionalization method that can attach a high dispersion of noble metal nanoparticles with clean surface onto CNT for high electrocatalytic activity.

Recently, Wei et al.³⁹ covalently functionalized CNT with thiol groups (SH-CNT) and highly dispersed Pt nanoparticles on SH-CNT, which displayed good durability and excellent activity in the oxygen reduction reaction. Niu and co-workers⁴⁰ grafted an ionic liquid onto CNT (CNT/IL) and uniformly deposited Au nanoparticles with an average diameter of 3.3 nm on the CNT/IL, showing significantly strengthened activity for oxygen reduction. These valuable tries inspired us to employ a controllable reaction under mild conditions, which would covalently graft massive and uniform distribution of functional groups on the CNT surface for dispersing noble metal nanoparticles with a clean surface for the electrocatalytic reaction. Herein, by selecting PtRu nanoparticles as the model because of the wide interest in their use in direct methanol fuel cells, we report a facile pathway for high-efficiency dispersion of noble metal nanoparticles on CNT. Our approach is based on the direct Friedel–Crafts reaction between pristine CNT and maleic anhydride using aluminium chloride as the catalyst to graft carboxyl-terminated groups onto pristine CNT (Scheme 1). The PtRu nanoparticle electrocatalysts with an average size of 3.3 nm and narrow size distribution were uniformly deposited on the resulting carboxylated-carbon nanotubes (CNT-C) in the following microwave-assisted polyol process. The grafted, high-density and uniform distribution carboxyl-terminated groups not only improve the hydrophilicity of the CNT to offer enough accessible specific surface area but also act as binding sites for anchoring metal ions and nanoparticles. More importantly, the short carboxyl-containing chain directly grafted on the carbon atoms of the CNT surface only gives rise to a negligible contact resistance between the PtRu nanoparticles and CNT, which does not affect the electron transport pathway. In addition, our method does not use any template agents, therefore the noble metal nanoparticles in the as-prepared nanohybrids have a clean surface for the electrocatalytic reaction. The resulting PtRu/CNT-C nanohybrids exhibit superior electrocatalytic activity for methanol oxidation when compared with PtRu nanoparticles supported on AO-CNT and previously reported PtRu/C catalysts.

Scheme 1 Schematic diagram of CNT-C and preparation of PtRu/CNT-C nanohybrids.

2. Experimental

2.1 Materials

Pristine multi-walled carbon nanotubes (CNT) (length 5–15 μm, diameter 20–60 nm) were purchased from Shenzhen Nanotech Port Co. Ltd., China. Maleic anhydride (99%) was purchased from Alfa Aesar. Other chemicals were of analytical grade and used as received.

2.2 Preparation of the CNT-C and PtRu/CNT-C catalysts

The preparation procedure for the CNT-C was as follows: pristine CNT (100 mg), maleic anhydride (1 g) was refluxed with AlCl₃ (1.33 g) in dried N-methyl-2-pyrrolidone at approximately 90 °C under a dry nitrogen atmosphere for 4 h. Then, the reaction mixture was stirred at 150 °C for 48 h. After the reaction, the mixture was decomposed with double-distilled water followed by 0.5 M HCl aqueous solution. The obtained samples were then washed five times with double-distilled water and then the pH adjusted to 9 using 1.0 M KOH aqueous solution. Finally, the filtered solid was dried under vacuum for 12 h at 40 °C to obtain CNT-C. The acid-oxidized CNT (AO-CNT) was prepared by refluxing pristine CNT in a mixed acid (H₂SO₄ : HNO₃ in a 1 : 3 v/v ratio) solution for 5 h and then washed with double-distilled water until the pH became neutral.

Deposition of the PtRu nanoparticles on CNT-C was carried out via a microwave-assisted reduction process in ethylene glycol. The details are as follows: 20 mg of CNT-C was mixed with 438 μL of H₂PtCl₆ (38.6 mM) in ethylene glycol. After ultrasonication for 15 min, 350 μL of RuCl₃ (48.2 mM) was added into the reaction mixture and ultrasonically blended for 30 min. Then, the pH value of the solution was adjusted to 8–9 using 1.0 M KOH aqueous solution. The mixture was placed in a microwave oven and heated at 120 °C with microwave irradiation (800 W) for 30 min. The products were centrifuged and washed three times with distilled water. The sample obtained was denoted as PtRu/CNT-C and dried in vacuum oven at 60 °C for 12 h. For comparison, PtRu nanoparticles supported on the AO-CNT, labeled as PtRu/AO-CNT, were prepared using the same procedure as described above.

2.3 Physical characterization of the electrocatalysts

The metal Pt and Ru loading mass for the PtRu/CNT-C (or PtRu/AO-CNT) catalyst was determined by inductively coupled plasma-atom emission spectroscopy (ICP-AES, Spectro Ciros) and the corresponding results shown in Table S1.† Fourier transform infrared spectrometry (FTIR, Nicolet 6700) and X-ray photoelectron spectrometry (XPS) (AXIS ULTRA, Kratos Analytical Ltd., Japan) were employed to analyze the surface chemical compositions of the CNT-C. The present FTIR study uses traditional KBr pellets and the pellets were prepared from a mixture of sample and KBr at a 1 : 100 (w/w) ratio. The Raman spectrum (in Via, Renishaw, England) was also used to study the integrity and electronic structure of the samples. The morphology and structure of the PtRu/CNT-C and PtRu/AO-CNT catalysts were characterized by transmission electron microscopy (TEM, JEOL 3010, 200 kV) and powder X-ray diffraction (XRD, Bruker AXS X-ray diffractometer), respectively.

2.4 Electrochemical measurements of the electrocatalysts

For electrochemical investigation, a glassy carbon (GC, 5 mm diameter) electrode was polished with the slurry of 0.5 and 0.03 μm alumina successively and washed ultrasonically in double-distilled water prior to use. The catalyst ink was prepared by dispersing 5 mg of catalyst in 5 mL of water by sonication. When a dark homogeneous dispersion was formed, 40 μL of the ink was dropped onto the GC electrode using a micro-syringe. After being dried in air, the electrode was coated with 10 μL of a 0.05 wt% Nafion ethanol solution to fix the catalyst powder. The electrochemical surface area (ESA) and the electrochemical performance of the electrocatalysts were evaluated by cyclic voltammetry. All electrochemical measurements were performed on a CHI660D electrochemical workstation (Chenhua Instrument Company of Shanghai, China). A conventional three-electrode glass cell was used with a platinum wire as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. All the potentials reported herein were with respect to SCE. Double-distilled water was used throughout.

3. Results and discussion

3.1 Characterization of the CNT-C support

Surface-functionalization of CNT was characterized using FTIR spectroscopy. The FTIR spectra of maleic anhydride, pristine CNT and CNT-C are shown in Fig. 1. In the FTIR spectrum of maleic anhydride, the characteristic band at 900 cm⁻¹ was assigned to the C–O–C group of MA (curve 1 in Fig. 1). The characteristic peak at 1596 cm⁻¹ corresponded to the v(C–H) stretching vibration and the peaks at 1850 and 1784 cm⁻¹, correspond to the asymmetric and symmetric vibration of v(C=O), respectively. The pristine CNT has no obvious characteristic absorption peaks because of their very few functional groups (curve 2 in Fig. 1). For the FTIR spectra of CNT-C (curve 3 in Fig. 1), there is no peak at 900 cm⁻¹, which confirms that the five membered rings are completely opened. The peaks at 1600 and 1417 cm⁻¹ are assigned to the asymmetric and symmetric stretching vibration of v(COO⁻) in the carboxylate ion. One broad peak at 3423 cm⁻¹ was ascribed to the v(O–H) stretching vibration of the absorption of water in the CNT-C. This result revealed the presence of carboxylate ions on the CNT-C and the successful functionalization of the CNT. The reaction mechanisms are as follows: (1) the maleic anhydride ring was completely opened by a Lewis acid catalyst (in this case, AlCl₃); (2) the carboxyl-terminated functional groups formed were then covalently grafted to the sidewalls of the CNT and reacted with the added KOH solution to form carboxylate ions. Moreover, the surface properties of PtRu/CNT-C were also characterized by FTIR spectroscopy (see the ESI, Fig. S1†). It is clear that the characteristic absorption peaks of the carboxylate ions appeared in the FTIR spectrum of PtRu/CNT-C, which indicated covalently grafted functional groups still exist on CNT-C surface after deposition of the PtRu nanoparticles.

Fig. 1 FTIR spectra of maleic anhydride (1), pristine CNT (2) and CNT-C (3).

On the other hand, XPS was also utilized to intuitively evaluate the concentration of oxygenated functional groups on CNT-C. As shown in Fig. 2a and b, the C1s peak (284.7 eV) and O1s peak (532.4 eV)⁴¹ can be clearly observed and the C/O ratio of CNT-C and AO-CNT were 31.9 and 43.1, respectively. The smaller C/O ratio of CNT-C means it has a higher concentration of oxygenated functional groups than AO-CNT. In the high-resolution C1s spectrum of the SNE-CNTs (Fig. 2c and d), the peak at 284.7 eV corresponds to sp²-hybridized graphitic carbon atoms. The small peaks at 285.9, 287.4, 289.1 and 291.3 eV obtained by peak fitting are assigned to the C–O, C=O, O–C=O and π–π* bonds, respectively.⁴¹ It is interesting to note that the CNT-C have a higher intensity of the O–C=O bond peak (289.1 eV) than AO-CNT, which indicated higher surface concentrations of carboxyl groups on the CNT-C.

Fig. 2 XPS wide scan spectra and curve fitting of the C1s peak of the FTIR spectra for CNT-C (a and c) and AO-CNT (b and d).

To analyze the structure of the resultant functionalized-CNT, the CNT-C and AO-CNT were further investigated using Raman spectroscopy (Fig. 3). It is noted that the peak at 1320 cm⁻¹ should be assigned to the A_1g breathing mode of the disordered graphite structure (i.e., the D band) and the peak at ∼1564 cm⁻¹ assigned to the E_2g structure mode of graphite (i.e., the G band).^32,38 The G band reflects the structure of the sp² hybridized carbon atoms. An additional side band at ∼1600 cm⁻¹ was also observed, which is assigned as the D′ band. Both the D and the D′ bands are due to the defect sites in the hexagonal framework of graphite materials. The extent of the defects in graphite materials can be quantified by the intensity ratio of the D to G bands (i.e., I_D/I_G).^3,12,32,38 It can be obtained from Fig. 2 that the values for the I_D/I_G ratio are 1.27, 1.36 and 1.65 for the pristine CNT, CNT-C and AO-CNT, respectively. The I_D/I_G value for the pristine CNT (1.27) is close to that reported in literature.³ It is noted that the values of the I_D/I_G ratio for both CNT-C and AO-CNT are higher than that of the pristine CNT because of the surface modification. However, the AO-CNT has a higher I_D/I_G ratio than the CNT-C, suggesting that the harsh chemical acid treatment causes more obvious structural damage of the CNT. The results from Raman spectra revealed that the Friedel–Crafts reaction process leads to less structural damage of the CNT than the typical acid-oxidized treatment due to its more-milder conditions. The CNT-C should save better electric conductivity than the AO-CNT and more suitable support noble metal nanoparticles for fuel cells.

Fig. 3 Raman spectra of AO-CNT (1), pristine CNT (2) and CNT-C (3).

As mentioned above, the excellent dispersibility of the CNT in solvents for offering enough accessible specific surface areas is one of the key issues for the uniform growth of metal nanoparticles. Here, a comparison of the dispersibility of CNT-C and pristine CNT in water was carried out and the corresponding results are shown in Fig. 4. Obviously, CNT-C can easily and uniformly disperse in water. In contrast, it is hard for pristine CNT to uniformly disperse in water with aggregation and precipitation being observed. This implies the dispersibility of CNT-C in water is greatly improved in comparison with pristine CNT, which results from the hydrophilic carboxylate ions interacting with water and so preventing the aggregation of CNT. The more hydrophilic materials will also lead to higher dispersion than hydrophobic ones for anchoring and growing more metal nanoparticles. It was expected that PtRu nanoparticles will be dispersed uniformly on the CNT-C with small particle size and narrow size distribution.

Fig. 4 Digital images of 10 wt% CNT-C (1) and pristine CNT (2) dispersed in water for one month.

3.2 Characterization of the PtRu/CNT-C catalyst

Fig. 5 shows the transmission electron microscopy (TEM) images of the PtRu/CNT-C and PtRu/AO-CNT nanohybrids. As shown in Fig. 4, CNT-C is successfully decorated with lots of well-dispersed PtRu nanoparticles. The TEM images with higher magnification of PtRu/CNT-C have also been provided (see the ESI, Fig. S2†). Their size distribution was evaluated statistically through measuring the diameter of 200 PtRu nanoparticles in the selected TEM images and the corresponding particle size distribution shown in Fig. S3.† It is noted that the particle size of the PtRu nanoparticles distributes mainly between 1.7 nm and 4.8 nm (with an average diameter of ca. 3.3 ± 0.5 nm). It is noteworthy that no nanoparticle aggregation is observed on the CNT surface. However, for the AO-CNT, the dispersion of PtRu nanoparticles is not satisfactory and has a broad distribution (1.5–9.5 nm) with an average diameter of ca. 5.5 ± 1.5 nm. This can be attributed to the following reasons: for the acid-oxidized CNT, only a limited and uneven distribution of carboxyl groups are generated on the defects sites of the CNT surface. When PtRu nanoparticles are deposited on the CNT surface, there are not enough and uniform carboxyl groups to anchor Pt and Ru precursors and PtRu nanoparticles, leading to poor dispersion and extensive aggregation of PtRu nanoparticles on the surface of CNT-AO. However, for the CNT-C, a large number of carboxyl groups with a uniform distribution are introduced by the direct Friedel–Crafts reaction that serve as the functional groups for immobilizing the Pt and Ru precursors on the CNT surface through electrostatic and coordination interactions. Therefore, PtRu nanoparticles on the surface of the CNT-C have a much more uniform distribution, which is a more effective catalyst support than CNT-AO.

Fig. 5 TEM images of the PtRu/CNT-C (a and b) and PtRu/AO-CNT (c and d) nanohybrids.

Fig. 6 shows the XRD patterns for PtRu nanoparticles deposited on CNT-C and AO-CNT. The diffraction peaks located at 25.0° originates from the graphitic carbon of the CNTs. The presence of diffraction peaks at 39.0°, 45.3°, and 66.7°, which can be assigned to Pt(111), Pt(200), and Pt(220), are consistent with the face-centered cubic (fcc) structure of platinum.³⁸ Moreover, there were no diffraction peaks for Ru or its oxides in the XRD patterns of samples, suggesting that the PtRu alloy nanoparticles are present in the fcc structure of Pt. The Pt(110) bands at 39.0° are broader and weaker for PtRu/CNT-C than that for PtRu/AO-CNT, indicating the smaller size of the PtRu nanoparticles on CNT-C. On the basis of Scherrer's equation^32,38 through line broadening of the Pt(220) peak, the average size of the PtRu nanoparticles for PtRu/CNT-C and PtRu/AO-CNT was calculated as 3.5 and 5.2 nm, respectively. These values agree with the TEM results.

Fig. 6 XRD patterns of the PtRu/CNT-C (1) and PtRu/AO-CNT (2) nanohybrids.

The electrochemical surface area (ESA) provides important information regarding the number of available electrochemical active sites on the catalysts surface and at the same time accounts for the access of a conductive path available to transfer electrons to and from the electrode surface. Fig. 7 shows the cyclic voltammograms (CVs) of the PtRu/CNT-C and PtRu/AO-CNT nanohybrids measured in a nitrogen-saturated 0.5 M H₂SO₄ solution. Well-defined CVs are obtained for all the PtRu catalyst samples. The cathodic and anodic peaks appearing between −0.25 and 0.10 V originate from the adsorption and desorption of atomic hydrogen in the acidic media. Fig. 7 clearly shows that PtRu/CNT-C have a higher integrated peak area than PtRu/AO-CNT, which is commonly used to give the mount of adsorption–desorption charges. Based on the hydrogen adsorption–desorption charges, the values of the ESA of PtRu nanoparticles supported on the CNT-C and AO-CNT could be calculated from:⁴²

ESA = Q_H/(0.21 × [Pt])

where Q_H (mC cm⁻²) represents the mean value between the amount of charge exchanged during the electro-adsorption and desorption of H₂ on Pt sites, [Pt] is the Pt loading (mg cm⁻²) on the electrode and 0.21 (mC cm⁻²) represents the charge required to oxidize a monolayer of H₂ on bright Pt. The results show that the ESA value of the PtRu/CNT-C catalyst (66.2 m² g⁻¹ Pt) is larger than that of the PtRu/AO-CNT catalyst (48.4 m² g⁻¹ Pt) and PtRu/C catalyst (16.5 m² g⁻¹ Pt) reported previously,³⁸ which is attributed to the smaller particle size and much better dispersion of PtRu nanoparticles on the CNT-C and the higher electron conductivity of CNT-C because of much lower defects. This also demonstrates that the PtRu nanoparticles deposited on the CNT-C are electrochemically more accessible, which is very important for the electrochemical oxidation of methanol.

Fig. 7 Cyclic voltammograms of the PtRu/CNT-C (1) and PtRu/AO-CNT (2) nanohybrids in a nitrogen-saturated 0.5 M H₂SO₄ aqueous solution at a scan rate of 50 mV s⁻¹.

The electrochemical performance of the PtRu/CNT-C and PtRu/AO-CNT catalysts towards methanol electrooxidation was examined using cyclic voltammetry in a nitrogen-saturated 0.5 M H₂SO₄ + 1.0 M CH₃OH aqueous solution and the corresponding results are shown in Fig. 8. A significant enhancement of the peak current of methanol oxidation can be observed on the PtRu/AO-CNT catalyst. It is noted that the onset potential of methanol oxidation shifts more than 100 mV in negative direction when the catalyst support is changed from the AO-CNT to the CNT-C. On the other hand, the mass activity of methanol oxidation on the PtRu/CNT-C catalyst is 496.3 mA mg⁻¹, which is 1.6 and 5.0 times higher than that on the PtRu/AO-CNT catalyst (304.1 mA mg⁻¹) and PtRu/C catalyst (98.3 mA mg⁻¹) reported previously.³⁸ In addition, the mass activity for methanol oxidation for the PtRu/CNT-C nanohybrids is higher than those of recent state-of-art Pt-based nanomaterials, such as PtRu/CNTs-PTCA,¹² Pt–PVP–MWCNTarc,³² PtRu/1-AP–MWCNTs,³⁸ and PtRu/CNTs–PIL.² The specific activity, defined by current normalizing to the ESA value of the catalysts, was also employed to further investigate the catalytic activity of the obtained nanohybrids (Fig. S4†). It was found that PtRu/CNT-C catalyst also had a higher specific activity (7.4 A m⁻²) than that of PtRu/AO-CNT catalyst (6.3 A m⁻²). In addition, as indicated by the dotted lines in Fig. S4,† the corresponding potential of PtRu/CNT-C is much lower than that of PtRu/AO-CNT, further demonstrating that PtRu/CNT-C at a given oxidation current density, has a significantly enhanced electrocatalytic activity.

Fig. 8 Cyclic voltammograms of the PtRu/CNT-C (1) and PtRu/AO-CNT (2) nanohybrids in a nitrogen-saturated 0.5 M H₂SO₄ + 1.0 M CH₃OH aqueous solution at a scan rate of 50 mV s⁻¹.

In practical applications, the long-term cycle stability of the catalysts is of great importance.⁴³ In this work, the long-term cycle stabilities of the PtRu/CNT-C and PtRu/AO-CNT catalysts have been investigated in a 0.5 M H₂SO₄ + 1.0 M CH₃OH aqueous solution using cyclic voltammetry and the corresponding results are shown in Fig. 9. From Fig. 9, it can be observed that the value of i_pI/i_pI(1) decreases gradually with the number of successive scans. In the case of PtRu/CNT-C nanohybrids, the peak current at the 600th cycle is about 89% of that measured at the first cycle. The decrease of i_pI/i_pI(1) on PtRu/CNT-C is only 11% for 600 cycles. However, for the PtRu/AO-CNT catalyst, a large decrease (27%) is found. To further eliminate the effects of the decrease of methanol concentration on the decay of the peak current, we replaced the methanol solution with fresh solution after 600 cycles and the recovery peak current was recorded (Fig. S5†). It is noted that for the PtRu/CNT-C electrocatalyst, the oxidation peak current can recover within 600 cycles and about a 6% loss occurs after 600 cycles. However, for the PtRu/AO-CNT electrocatalyst, the recovery of the peak current can be observed as an 18% loss after 600 cycles. The improved durability of the PtRu/CNT-C catalyst was attributed to the high density of functional groups, the structural integrity and high corrosion resistance of CNT-C, which helps to inhibit PtRu nanoparticles dissolution, ripening and aggregation.

Fig. 9 Long-term cycle stability of the PtRu/CNT-C (1) and PtRu/AO-CNT (2) nanohybrids in a nitrogen-saturated 0.5 M H₂SO₄ + 1.0 M CH₃OH solution, where i_pI is the forward peak current and i_pI(1) the forward peak current at the first cycle.

4. Conclusions

In summary, we have successfully developed a simple synthetic strategy for the noble metal nanoparticles/CNT nanohybrids with high electrocatalytic activity based on the carboxylated-carbon nanotubes. Due to the introduction of carboxylate ion groups on the CNT surface with high density and uniform distribution, the solubility of CNT in water and its available specific surface areas have been significantly improved. When compared to the PtRu/AO-CNT nanohybrids, PtRu/CNT-C nanohybrids display significantly enhanced electrocatalytic activity and stability towards methanol oxidation because of the smaller particle size and higher dispersion of the PtRu nanoparticles on CNT-C, and stronger interactions between the PtRu nanoparticles and CNT-C. The CNT-C is a promising catalyst support for noble metal nanoparticles in fuel cells.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (21303134), China Postdoctoral Science Foundation (2013M532017) and Natural Science Foundation of Shaanxi Province, China (2014JQ2046).

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Footnote

† Electronic supplementary information (ESI) available: ICP-AES analysis of the electrocatalysts (Table S1); FTIR spectra of PtRu/CNT-C (Fig. S1); TEM images of the PtRu/CNT-C nanohybrids (Fig. S2); size distribution of the PtRu nanoparticles for the PtRu/CNT-C and PtRu/AO-CNT nanohybrids (Fig. S3); cyclic voltammograms (specific activity) and linear sweep voltammetry for the PtRu/CNT-C and PtRu/AO-CNT nanohybrids in a nitrogen-saturated 0.5 M H₂SO₄ + 1.0 M CH₃OH aqueous solution at a scan rate of 50 mV s⁻¹ (Fig. S4 and S5); comparison of the forward peak current at the first cycle (i₀) and recovery forward peak current (i_R) in the fresh methanol solution after long-term cyclic voltammograms scanning experiments (600 cycles) for the PtRu/CNT-C and PtRu/AO-CNT nanohybrids (Fig. S6). See DOI: 10.1039/c4ra14659j

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