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MWCNTs nanocomposites for methanol electro-oxidation
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Electrochimica Acta 53 (2008) 4316–4323
Hydrothermal synthesis of size-dependent Pt in Pt/MWCNTs nanocomposites for methanol electro-oxidation Liang Chen a,b , Gongxuan Lu a,∗ a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China b Graduate University of the Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100039, PR China Received 30 October 2007; accepted 24 December 2007 Available online 8 January 2008
Abstract A hydrothermal method has been developed to prepare size-controlled Pt nanoparticles dispersed highly on multiwalled carbon nanotubes (Pt/MWCNTs). It was found that the size of Pt nanoparticles was strongly dependent on the solution pH in synthesis. The Pt nanoparticles with mean size of 3.0, 4.2 and 9.1 nm were obtained at pHs 13, 12 and 10 separately. After Pt/MWCNTs composites were fabricated, the different properties of cyclic voltammetry and chronoamperometry in electro-oxidation of methanol were found. The results showed that the smaller diameter Pt deposited Pt/MWCNTs nanocomposites exhibited higher electrocatalytic activity for methanol oxidation. By characterization of X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), size-dependent activities were identified. © 2008 Elsevier Ltd. All rights reserved. Keywords: Size-dependent; Pt nanoparticles; Multiwalled carbon nanotubes; Methanol electro-oxidation
1. Introduction To meet world ever-increasing energy needs and deal with the serious environmental problems, one has to find the alternative energy-utilization method other than traditional ways. Direct methanol fuel cells (DMFCs) are one of the most promising power-utilization ways for application in portable device, vehicles and small stationary, due to their high power density, relatively quick startup and low operating temperature [1,2]. In this electrochemical cell, methanol is directly oxidized with air to carbon dioxide and water to produce electricity. Up to date, Pt catalyst has been the most frequently used catalytic material for DMFCs at room temperature [3–15]. Although it is well-known that the catalytic activity of the metal is strongly dependent on their shape, size and distribution, the detailed investigation of this difference is not very clear. Due to the limited supply of Pt and the potential widespread commercialization of fuel cell, it
∗
Corresponding author. Tel.: +86 931 4968178; fax: +86 931 4968178. E-mail address: [email protected] (G. Lu).
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.12.076
is urgent to develop method to fabricate a highly dispersed Pt catalyst on supports in an easy way. Recently, it was reported that this aim might be fulfilled by wet impregnation [9], physical evaporation [16], electrodeposition [17], microwave-assisted polyol strategy [18], supercritical fluid method [19] and pseudomicroemulsion [20]. But these methods were often complex, entail environmental cost or require expensive instruments. In addition, some techniques do not provide adequate control of particle shape and size. In the present work, we report a hydrothermal process to prepare size-controlled and well-dispersed Pt nanoparticles on multiwalled carbon nanotubes (MWCNTs). By adjusting the synthesis solution pH, Pt/MWCNTs nanocomposites with selected Pt particle size could be obtained. Effect of the particle size of Pt/MWCNTs nanocomposite on the electrooxidation of methanol was studied. It was shown that the Pt/MWCNTs nanocomposites prepared under higher pH exhibited higher electrocatalytic activity for methanol oxidation. This demonstration may pave a new way to prepare homogeneous and well-defined nanoparticle catalysts for fuel cell application.
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2. Experimental 2.1. Hydrothermal synthesis of Pt/MWCNTs nanocomposites MWCNTs (purity 95%, diameter 20–60 nm) purchased from Shenzhen Nanotech Port. Co., Ltd (Shenzhen, China) were used as received. Before the deposition of Pt nanoparticle, MWCNTs were functionalized with –C O, –COO and –C–OH groups by refluxing in a 2.6 M nitric acid solution at about 123 ◦ C for 24 h [21]. Pt nanoparticles supported on MWCNTs were synthesized by the hydrothermal technique. A typical preparation consisted of the following steps: 2.7 mL of an aqueous solution of 0.03862 M H2 PtCl6 and 80 mg of purified MWCNTs were added into 10 mL of glycerol (or ethylene glycol (EG)) in a 100 mL vessel. The aqueous NaOH solution was then added, dropwise, to a desired pH value. Then the mixture was ultrasonicated for 30 min to ensure the uniform dispersion of MWCNTs in the solution. Seventy-five milliliters of mixture solution was transferred into a 100 mL Teflon-lined autoclave and maintained at 160 ◦ C for 5 h. Finally, the product was collected through filtration and washed with ethanol and deionized water, and then dried at 373 K for 12 h. The electrocatalysts were labeled as Pt/MWCNTs-1, Pt/MWCNTs-2 and Pt/MWCNTs-3 for pHs 13, 12 and 10, respectively. The Pt/MWCNTs nanocomposite prepared from the ethylene glycol solution of H2 PtCl6 with pHs 13 was referred to as Pt/MWCNTs-4. 2.2. Physical characterization The size of various Pt/MWCNTs nanocomposites was determined on a transmission electron microscope (JEOL-1200EX). For microscopic examinations, the samples were first ultrasonicated in water and then were deposited on Cu grids covered with a continuous carbon film. X-ray photoelectron spectroscopy (XPS) was performed on a VG Escalab 210 electron spectrometer with Mg K␣ radiation. The base pressure of the system was 2 × 10−8 Pa, and the measurements were carried out from 8 × 10−7 to 1 × 10−6 Pa. For each catalyst, a survey spectrum was collected before high-resolutions were recorded. The binding energy of the C 1s of graphite (284.5 eV) was taken as reference. A Philips X’Pert MPD instrument with Cu K␣ radiation was used to obtain the powder X-ray diffraction (XRD) patterns.
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using a CHI660A electrochemical workstation (CH Instruments, Shanghai, China). A three-electrode cell was used with a saturated calomel electrode (SCE) as a reference electrode, a platinum wire as a counter electrode, and a glassy carbon electrode as a working electrode. Before the measurement, all working electrodes were completely activated to a steady state by cyclic voltammetry (20 cycles in 0.5 M H2 SO4 solutions at a scan rate of 100 mVs−1 ). The electrolyte for electrochemical measurements was a solution of 1 M methanol in 0.5 M H2 SO4 . The solution was deaerated with ultrahigh-purity N2 before scanning. The current density was normalized to the geometric area of electrode, unless otherwise specified. 3. Results and discussion 3.1. Characterization of Pt/MWCNTs nanocomposites MWCNTs are considered to be a more attractive candidate due to their extraordinary mechanical characteristics such as high tensile strength coupled with high surface area, high electric conductivity, and thermal conductivity. Pt can be deposited on the MWCNTs by the oxidation of glycerol. In comparison with other reducing reagents such as formic acid [6], ethylene glycol [18], formaldehyde [20], sodium borohydride [22] and hydrazine [23], glycerol is cheaper and environmentally friendly. Here glycerol was used as a reducing reagent and stabilizer. The XRD patterns of Pt/MWCNTs nanocomposites prepared by hydrothermal method are shown in Fig. 1. There is one sharp reflection at about 2θ = 26◦ and three weak reflections at about 2θ = 42.8◦ , 53.8◦ and 77.8◦ that can be indexed to the carbon nanotubes. The diffraction peaks at 2θ of 39.6◦ , 46.2◦ , 67.4◦ and 81.5◦ can be attributed to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) reflection of face-centered cubic (f.c.c.) crystal lattice of platinum. From the full width half maximum of the (2 2 0) peak, the volume averaged particle size was calculated using the Scherrer equation [24]: L=
0.9λk␣1 (B2θ cos θB )
2.3. Electrochemical measurement Electrochemical activity of the synthesized Pt/MWCNTs nanocomposites was evaluated using a glassy carbon electrode (GCE, diameter 3 mm). Prior to coating, GCE was polished sequentially with slurries of 0.3 and 0.05 m alumina to mirror finish. After rinsing with redistilled water, it was cleaned ultrasonically in water for 10 min. To prepare the working electrode, 4 mg of Pt/MWCNTs was dispersed in 1 mL of diluted Nafion solution (0.5 wt%), and 6 L of the suspension was pipetted on to a GCE. The total Pt loading was 68 g cm−2 . Electrochemical measurements were performed
Fig. 1. XRD pattern of Pt/MWCNTs-1 (curve a), Pt/MWCNTs-2 (curve b) and Pt/MWCNTs-3 (curve c) and Pt/MWCNTs-4 (curve d) nanocomposites.
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where L is the average particle size, λk␣1 is the X-ray wave length ˚ for Cu k␣1 radiation), B2θ is the peak broadening, (1.54056 A and θ B is the angle corresponding to the peak maximum. In the obtained samples, the average sizes of Pt nanoparticles are 3, 4.2 and 9.1 nm for pHs 13, 12 and 10, respectively. The results indicate that the size of Pt nanoparticles deposited on MWCNTs is determined by the pH of solution. It was also found that the mean size of Pt nanoparticles is 6.7 nm in the presence of ethylene glycol instead of glycerol. Fig. 2 shows the transmission electron microscopy (TEM) images of various Pt/MWCNTs nanocomposites prepared from the glycerol solution with different pHs. As shown in Fig. 2, the size of Pt particle deposited on MWCNTs surface decreases with the increase of glycerol solution pH, which is in accordance with the XRD results. The average sizes of Pt particles are 2.9, 4.4 and 10.1 nm for pHs 13, 12 and 10, respectively. In the case of pHs 12 and 10, Pt nanoparticles are apparently agglomerated and not well dispersed on MWCNTs, and the particle size ranges from 3 to 15 nm. In the case of pH 13, Pt nanoparticles are hardly agglomerated and well dispersed on the surface of MWCNTs. The above facts indicated that the increase of glycerol solution pH could improve the size uniformity and dis-
persion of Pt nanoparticles on the surface of MWCNTs. Similar phenomenon were also observed by other groups [25–28]. TEM image of Pt/MWCNTs-4 nanocomposites was compared. It is noticed that Pt nanoparticles reduced by ethylene glycol are more easily agglomerated than those reduced by glycerol. The average particle size is 7.1 nm. The chemical composition of the Pt nanoparticles deposited on functionalized MWCNTs was analyzed by XPS. Fig. 3 displays the XPS spectra of various Pt/MWCNTs nanocomposites prepared by the hydrothermal method. The chemical composition of the Pt nanoparticles deposited on functionalized MWCNTs was analyzed by XPS. Fig. 3a displays the narrow scan of Pt (4f) core level of XPS spectra of various Pt/MWCNTs nanocomposites prepared by the hydrothermal method. The spectrum shows a doublet containing a low energy band (4f7/2 ) at about 71.0 eV, and a high energy band (4f5/2 ) centered at 74.5 eV. The area ratio between 4f7/2 and 4f5/2 components is 4/3. Deconvolution of the Pt 4f7/2 region of Pt/MWCNTs-1 shows the presence of two pairs of doublets, which indicates the existence of two different Pt oxidation states (see Fig. 3b). The doublet with binding energy of 70.1 eV was attributed to metallic Pt. Peaks at 72.3 eV could be assigned to Pt(II) as in either PtO
Fig. 2. TEM images of Pt/MWCNTs-1 (a), Pt/MWCNTs-2 (b), Pt/MWCNTs-3 (c) and Pt/MWCNTs-4 (d) nanocomposites.
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Fig. 4. Cyclic voltammograms of Pt/MWCNTs-1 (curve a), Pt/MWCNTs-2 (curve b) and Pt/MWCNTs-3 (curve c) nanocomposites in nitrogen saturated 0.5 M H2 SO4 solutions. Scan rate: 50 mVs−1 .
3.2. Active specific surface area of Pt nanoparticles
Fig. 3. (a) XPS spectra of Pt/MWCNTs-1 (curve a), Pt/MWCNTs-2 (curve b), Pt/MWCNTs-3 (curve c) and Pt/MWCNTs-4 (curve d) nanocomposites. (b) A typical example of XPS Pt 4f peak deconvolution of Pt/MWCNTs-1.
or Pt(OH)2 [7,14]. The percentages of these two species for Pt/MWCNT-1 are calculated from the relative intensity of these peaks are given as 73.9 and 26.1%, respectively (see Fig. 3b). The detailed binding energies and relative intensities of different Pt species on other Pt/MWCNTs catalysts were given in Table 2. It was observed that the content of Pt(0) decreases and the content of Pt(II) increases with increasing the average size of the Pt particles.
The voltammograms of various Pt/MWCNTs nanocomposites prepared under different pHs in 0.5 M H2 SO4 solution are shown in Fig. 4. For the Pt/MWCNTs-3 catalysts, a broad peak with slight current density is observed in the hydrogen adsorption–desorption region. This is probably due to agglomeration and poor dispersion of the Pt nanoparticles in this catalyst. Conversely, for the Pt/MWCNTs-1 and Pt/MWCNTs-2 catalysts, well-defined hydrogen adsorption–desorption peaks with a much lager area are observed in the potential region −0.25 to 0.05 V, demonstrating the higher surface area of above both catalysts. The high surface area is owing to the presence of narrow size and uniform distribution of the Pt nanoparicles as displayed in the TEM images. In general, the real surface area of Pt nanoparticles is an important parameter to determine the catalytic properties of electrocatalysts for methanol electrooxidation since this reaction is surface-sensitive. The areas in m2 g−1 were calculated from the following formula assuming a corresponding value of 0.21 mC cm−2 (calculated from the surface density of 1.3 × 1015 atom per cm2 , a value generally admitted for polycrystalline Pt electrodes) and the Pt loading [29]. m2 QH = AEL −3 (g catalyst) (0.21 × 10 C(g catalyst))
Table 1 Average crystallite size of Pt nanoparticles, active specific area, peak current density and specific current density for methanol oxidation reaction of various Pt/MWCNTs nanocomposites Pt/MWCNTs nanocomposites
d (nm)
QH (mC)
SEL (cm−2 )
AEL (m2 g−1 )
PCD (mA cm−2 )
SCD (mA cm−2 )
Pt/MWCNTs-1 Pt/MWCNTs-2 Pt/MWCNTs-3 Pt/MWCNTs-4
3.0 4.2 9.1 6.7
1.28 1.14 0.30 0.58
4.6 4.1 1.1 2.1
95.8 85.4 22.3 43.8
43 33 4 21
0.66 0.57 0.26 0.71
d: the average size of Pt nanoparticles was obtained by XRD analysis; QH : the amount of charges exchanged during the electroadsorption of hydrogen on Pt nanoparticles; SEL : the specific surface area of Pt nanoparticles obtained electrochemically; AEL : the specific surface area of Pt nanoparticles obtained electrochemically; PCD: peak current density at 0.64 V for methanol oxidation reaction which was normalized to the geometric area of electrode; SCD: specific current density at 0.64 V for methanol oxidation reaction in terms of the real surface area of electrode.
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Electrochemical surface areas of various Pt/MWCNTs nanocomposites are listed in Table 1. The specific surface area of Pt nanoparticles are 95.3, 85.4 and 22.3 m2 g−1 for pHs 13, 12 and 10, respectively. It shows that the real surface area of Pt nanoparticles increases with the increase of synthesis solution pH. This is not surprising in view of the decrease of the metallic particle size with the pH increasing. 3.3. Electrocatalytic activity of Pt/MWCNTs nanocomposites prepared under different pHs Cyclic voltammetry is a valuable and convenient tool for studying methanol oxidation catalysts. The voltammograms of methanol oxidation over Pt/MWCNTs nanocomposites prepared from the glycerol solution with different pHs were very similar to that of the Pt/C catalysts (see Fig. 5). In the forward scan, the current of methanol oxidation becomes apparent as the potential rises about 0.35 V, and methanol oxidation produced a prominent symmetric anodic peak around 0.64 V. As shown in Table 1, the specific catalytic activity of the Pt/MWCNTs nanocomposite for the methanol oxidation follows the order Pt/MWCNTs-1 > Pt/MWCNTs-2 > Pt/MWCNTs-3. It is indicated that the electrocatalytic activity of Pt/MWCNTs catalysts for the methanol oxidation reaction depends strongly on the particle size of dispersed Pt nanoparticles. In the reverse scan, an anodic peak current density was detected at around 0.42 V. It could be attributed to the removal of the incompletely oxidized carbonaceous species formed in the forward scan. The residual carbon species may be oxidized according to the following reaction: PtOHad + Pt C O → CO2 + 2Pt + H+ + e− Hence the ratio of the forward anodic peak current density (If ) to the reverse anodic peak current density (Ib ), If /Ib , can be used to describe the catalyst tolerance to carbonaceous species accumulation. Low If /Ib ratio indicates poor oxidation of
Fig. 5. Cyclic voltammograms of methanol electro-oxidation over the Pt/MWCNTs-1 (curve a), Pt/MWCNTs-2 (curve b) and Pt/MWCNTs-3 (curve c) nanocomposites in 1 M CH3 OH/0.5 M H2 SO4 electrolyte at 50 mVs−1 at room temperature.
methanol to carbon dioxide during the anodic scan and excessive accumulation of carbonaceous residues on the catalyst surface. High If /Ib ratio shows the converse case. Experimentally, the ratios were 1 and 1.1 for Pt/MWCNTs-1 and Pt/MWCNTs-2, respectively. Such a high value indicates that a large amount of intermediate carbonaceous species was oxidized to carbon dioxide in the forward scan. For comparison, the ratio 0.87 was reported with a nanosized Pt on XC-72 synthesized by a microwave-assisted polyol process [6]. These experimental observations highlighted the high activity for methanol oxidation of Pt/MWCNTs nanocomposites prepared by this hydrothermal method. However, the Pt/MWCNTs nanocomposites prepared under pH 10 exhibits slight catalytic activity toward methanol electro-oxidation. It is also found that Pt/MWCNTs nanocomposites prepared under pH < 10 exhibits little catalytic activity (data not shown). It shows that the synthesis solution pH plays an important role in the size of Pt nanoparticles, which is essential for the electroactivity toward methanol oxidation. Fig. 6 shows the change of polarization current densities with time of methanol electro-oxidation on various Pt/MWCNTs catalysts at 0.4 V versus SCE. The current density for the Pt/MWCNTs nanocomposites decayed with time and reached an apparent steady state within 300 s. The polarization current density of Pt/MWCNTs at all corresponding potentials also follows the order Pt/MWCNTs-1 > Pt/MWCNTs-2 > Pt/MWCNTs-3, which is in accordance with the cyclic voltammetry result. The above results clearly show that the specific activity enhanced with the decrease of size of Pt nanoparticles. But there have been contradictory conclusions regarding the influence of size and surface structure of these nanoparticles on the electrocatalytic activity. Some investigators have found that the specific activity with a decrease in platinum particle size, but others have observed different results [30–32]. Stonehart and Watanabe have insisted that the crystallite size effect is not truly dependent on the dimensions of the platinum crystallites but is dependent on the intercrystallites distance [33,34]. According to their hypoth-
Fig. 6. Polarization current densities versus time plots for the electro-oxidation of methanol on Pt/MWCNTs-1 (curve a), Pt/MWCNTs-2 (curve b) and Pt/MWCNTs-3 (curve c) in 1 M CH3 OH/0.5 M H2 SO4 electrolytes at 0.4 V (versus SCE) at room temperature.
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Table 2 Binding energies and relative intensities of different platinum species as observed from Pt (4f7/2 ) XPS of various catalysts Sample
d (nm)
Pt 4f 7/2 peak Position (eV)
Species
Pt 4f7/2 binding energy (eV)
Relative intensity (%)
Pt/MWCNTs-1
3.0
71.03
Pt(0) Pt(II)
70.98 72.30
73.9 26.1
Pt/MWCNTs-2
4.2
70.98
Pt(0) Pt(II)
70.90 71.85
58.5 41.5
Pt/MWCNTs-3
9.1
70.62
Pt(0) Pt(II)
70.61 71.47
37.1 62.9
Pt/MWCNTs-4
6.7
70.87
Pt(0) Pt(II)
70.76 71.79
58.4 41.6
esis, therefore, the specific activity will increase with a decrease in the size of platinum particles if the crystallite separation is shorter than ca. 18 nm. On the other hand, many other investigators have found no such result for the reaction but instead they have found that the specific activity steeply decreased with the decreasing size of platinum particles [35–37]. The essence of size effect of Pt nanoparticles on methanol electro-oxidation is not very clear and needs further investigation. It is generally accepted that the crucial and rate limiting step in methanol electro-oxidation reaction is the oxidation of a methanol adsorbate by an adsorbed hydroxy species or activated water group. When it is assumed that –COH is the methanol adsorbate, the rate-determining step is Pt–COH + Pt–OH → 2Pt + CO2 + 2H+ + 2e−
(1)
The rate of reaction (1) depends on coverage with θ C–OH and θ OH . Since θ C–OH + θ OH < 1, it implies that a high θ OH or θ C–OH will decrease the number of Pt sites that are available for the formation of an adsorbed methanol or hydroxy species, respectively, and the reaction rate will consequently be low. Hence it is obvious that the efficiency of the Pt catalyst would be improved if a well balanced co-adsorption of methanol and water could be realized at low potential. It was observed that the shift to higher potentials for the Pt oxide reduction with increasing particle size in Fig. 4. Also, the onset of oxidation is shifted to higher potentials with increasing particle size. This is indicative of a lower Pt–O affinity for larger Pt nanoparticles and implies that the ability to activate water, i.e. to break H–OH bonding with the formation of a Pt–OH species, proceed at a lower potential for small particles. Hence, it was assumed that the smaller sized catalysts contain more Pt–OH groups at a given potential than larger ones. In view of reaction (1), it was expected that an increase in specific activity for the methanol oxidation with a decrease in particle size. On the other hand, the intensity of Pt oxides enhanced with the increase of the particle size, as shown in Table 2. It was implied that the smaller particles leave fewer Pt sites for methanol adsorbates. Owning to the smaller amount of the methanol adsorbates, the specific activity of the large Pt particles is low. The above two reasons could imply that Pt/MWCNTs nanocomposites with smaller diameter Pt nanoparticles exhibited better intrinsic activity.
3.4. Electro-oxidation of methanol over Pt/MWCNTs-1 nanocomposites The effect of the scan rate on the electro-oxidation of methanol over Pt/MWCNTs-1 nanocomposites is shown in Fig. 7. In the range of 20–200 mVs−1 , the peak current density increases linearly with the square root of the scan rates. It shows that the methanol electro-oxidation process on the Pt/MWCNTs1 nanocomposites is controlled by the diffusion of methanol to the electrode surface. Additionally, the peak potential (the forward scan) increases with the increase of scan rate. It is indicated that the oxidation of methanol is an irreversible electrode process. Fig. 8 shows the effect of methanol concentration on the oxidation reaction for Pt/MWCNTs-1 nanocomposites in 0.5 M H2 SO4 at room temperature. Anodic peak current density increased with the concentration of methanol in the region of 0.1–4 M. However, a saturation of the current response was observed at methanol concentration of 6 M. The oxidation of methanol produces CO and possibly other adsorbed intermediates, which can be oxidized with the participation of oxygenated species formed on the surface. The latter process is inhibited by the adsorption of methanol on the surface, which increases
Fig. 7. Cyclic voltammograms of methanol electro-oxidation on the Pt/MWCNTs-1 nanocomposites in 1 M CH3 OH/0.5 M H2 SO4 electrolyte at various scan rates at room temperature. Scan rate (from inner to outer): 20, 40, 60, 80, 100, 150 and 200 mVs−1 .
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Fig. 8. Linear voltammograms of methanol oxidation on Pt/MWCNTs-1 nanocomposites at 50 mVs−1 in (a) 0.1 M, (b) 0.5 M, (c) 1 M, (d) 2 M, (e) 3 M, (f) 4 M CH3 OH/0.5 M H2 SO4 electrolyte.
Fig. 10. Cyclic voltammograms of Pt/MWCNTs nanocomposites prepared from the ethylene glycol solutions of H2 PtCl6 with pH 13 in the presence of MWCNTs in (a) nitrogen saturated 0.5 M H2 SO4 and (b) nitrogen saturated 0.5 M H2 SO4 + 1 M CH3 OH. Scan rate: 50 mVs−1 .
with the increase in methanol concentration. Cyclic voltammetry also detected a slow but gradual current density decrease with cycling, which is often attributed to the accumulation of strong chemisorbed species on the catalyst surface. The effect of the potential scan limit on the peak current density is shown in Fig. 9. The forward scan peak current density increases with increasing the anodic limit in the forward scan. The reverse anodic peak potential shifted negatively with increasing the anodic limit in the forward scan, while the peak current density decreases. This behavior indicates that the reverse anodic peak current density is primarily associated with residual carbon species on the surface of the Pt/MWCNTs-1 electrode.
and 0.5 M H2 SO4 + 1 M CH3 OH solutions are presented in Fig. 10. For this catalyst, the specific surface area is 43.8 m2 g−1 . The peak current density at 0.64 V was 21 mA cm−2 , which is one half of the value of 43 mA cm−2 obtained for the reaction on Pt/MWCNTs-1. It is indicated that Pt/MWCNTs-1 possesses better electrocatalytic activity toward methanol oxidation compared to Pt/MWCNTs-4. At high temperature, glycerol exhibits strong reducing ability, especially in alkaline condition. H2 PtCl6 could be readily reduced to Pt nanoparticle by glycerol at 160 ◦ C. The possible reaction pathway could be illustrated by following equation:
3.5. Comparison of electrocatalytic activity of Pt/MWCNTs nanocomposites synthesized by different reducing agents The cyclic voltammograms of Pt/MWCNTs nanocomposites prepared by ethylene glycol reduction method in 0.5 M H2 SO4
Fig. 9. Cyclic voltammograms of room temperature methanol electro-oxidation over Pt/MWCNTs-1 nanocomposites at 50 mVs−1 in 1 M CH3 OH/0.5 M H2 SO4 electrolyte for different forward potential scan limits.
C3 H5 (OH)3 + 3PtCl6 2− + 16OH− → 18Cl− + CO3 2− + C2 O4 2− + 12H2 O + Pt For this reaction pathway to take place, the –OH groups of glycerol interact with Pt-ion sites, resulting in the oxidation of glycerol to oxalate and carbonate in alkaline media. Under alkaline condition, small and uniform Pt nanoparticles deposited on MWCNTs could be prepared by above hydrothermal method. There are two reasons for it. One is that functionalized MWCNTs could interrupt metal particles agglomeration due to their anchorage on MWCNTs surface. It is known that nitric acid treatment could introduce carboxylic acid groups of the open ends and defect sites of the MWCNTs. At strong alkaline condition, a high intensity of carboxylate on the surface of MWCNTs was reached. These surface groups could act as anchoring sites and/or nucleating sites for nanoclusters in the reduction step of Pt precursor, and therefore prevent Pt particles from agglomerating. On the other hand, oxalate is believed to be act as a stabilizer for the Pt colloids, possibly forming chelate-type complexes via its carboxyl groups [26,38]. Such interactions between the Pt colloids and acid form (oxalic acid) are smaller. Oxalic acid is thus believed to be poor stabilizer for noble metal nanoparticle. Therefore, the pH of the synthesis solution is expected to greatly influence the stability and size of the metal nanoparticles. On the basis of the above, Pt nanoparticles prepared with low pH should
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be unstable and easily agglomerated, thus have large size and wide size distribution. With the increase of the synthesis solution pH, the Pt nanoparticles would become smaller and more uniform, as shown in Fig. 1a–c. In comparison with EG, smaller and more uniform Pt nanoparticles supported on MWCNTs were reduced by glycerol in hydrothermal environment. It could be due to the fact that oxalate is a good stabilizer for Pt colloid and glycerol has more hydroxyl groups that could prevent Pt colloids from agglomerating. 4. Conclusions In summary, Pt nanoparticles deposited on MWCNTs were prepared by a facile hydrothermal method. Glycerol was utilized as a reducing reagent and stabilizer. The dispersion and size of Pt nanoparticles in Pt/MWCNTs nanocomposites are remarkably affected by the synthesis solution pH. It is clarified that Pt/MWCNTs nanocomposites with smaller size of Pt nanoparticles exhibited higher electrocatalytic activity toward methanol electro-oxidation by cyclic voltammetry and chronoamperometry. The essence of the size-dependent activities of Pt/MWCNTs toward methanol electro-oxidation was also discussed. Acknowledgements This work was supported by the National Natural Science Foundation of China (Project number, 90210027) and the China Nation Key Basic Research Special Funds (Project numbers, 2003CB214500, and 2007CB613305). References [1] M. Winter, R.J. Brodd, Chem. Rev. 104 (2004) 4245. [2] B.D. McNicol, D.A.J. Rand, K.R. Williams, J. Power Sources 83 (1999) 15. [3] K.Y. Chan, J. Ding, J. Ren, S. Cheng, K.Y. Tsang, J. Mater. Chem. 14 (2004) 505. [4] F. Bensebaa, A.A. Farah, D. Wang, C. Bock, X. Du, J. Kung, Y. Le Page, J. Phys. Chem. B 109 (2005) 15339. [5] S. Liao, K.A. Holmes, H. Tsaprailis, V.I. Birss, J. Am. Chem. Soc. 128 (2006) 3504. [6] Z. Liu, X.Y. Ling, X. Su, J.Y. Lee, J. Phys. Chem. B 108 (2004) 8234. [7] J. Prabhuram, X. Wang, C.L. Hui, I.M. Hsing, J. Phys. Chem. B 107 (2003) 11057.
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Electrochimica Acta 53 (2008) 4316–4323
Hydrothermal synthesis of size-dependent Pt in Pt/MWCNTs nanocomposites for methanol electro-oxidation Liang Chen a,b , Gongxuan Lu a,∗ a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China b Graduate University of the Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100039, PR China Received 30 October 2007; accepted 24 December 2007 Available online 8 January 2008
Abstract A hydrothermal method has been developed to prepare size-controlled Pt nanoparticles dispersed highly on multiwalled carbon nanotubes (Pt/MWCNTs). It was found that the size of Pt nanoparticles was strongly dependent on the solution pH in synthesis. The Pt nanoparticles with mean size of 3.0, 4.2 and 9.1 nm were obtained at pHs 13, 12 and 10 separately. After Pt/MWCNTs composites were fabricated, the different properties of cyclic voltammetry and chronoamperometry in electro-oxidation of methanol were found. The results showed that the smaller diameter Pt deposited Pt/MWCNTs nanocomposites exhibited higher electrocatalytic activity for methanol oxidation. By characterization of X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), size-dependent activities were identified. © 2008 Elsevier Ltd. All rights reserved. Keywords: Size-dependent; Pt nanoparticles; Multiwalled carbon nanotubes; Methanol electro-oxidation
1. Introduction To meet world ever-increasing energy needs and deal with the serious environmental problems, one has to find the alternative energy-utilization method other than traditional ways. Direct methanol fuel cells (DMFCs) are one of the most promising power-utilization ways for application in portable device, vehicles and small stationary, due to their high power density, relatively quick startup and low operating temperature [1,2]. In this electrochemical cell, methanol is directly oxidized with air to carbon dioxide and water to produce electricity. Up to date, Pt catalyst has been the most frequently used catalytic material for DMFCs at room temperature [3–15]. Although it is well-known that the catalytic activity of the metal is strongly dependent on their shape, size and distribution, the detailed investigation of this difference is not very clear. Due to the limited supply of Pt and the potential widespread commercialization of fuel cell, it
∗
Corresponding author. Tel.: +86 931 4968178; fax: +86 931 4968178. E-mail address: [email protected] (G. Lu).
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.12.076
is urgent to develop method to fabricate a highly dispersed Pt catalyst on supports in an easy way. Recently, it was reported that this aim might be fulfilled by wet impregnation [9], physical evaporation [16], electrodeposition [17], microwave-assisted polyol strategy [18], supercritical fluid method [19] and pseudomicroemulsion [20]. But these methods were often complex, entail environmental cost or require expensive instruments. In addition, some techniques do not provide adequate control of particle shape and size. In the present work, we report a hydrothermal process to prepare size-controlled and well-dispersed Pt nanoparticles on multiwalled carbon nanotubes (MWCNTs). By adjusting the synthesis solution pH, Pt/MWCNTs nanocomposites with selected Pt particle size could be obtained. Effect of the particle size of Pt/MWCNTs nanocomposite on the electrooxidation of methanol was studied. It was shown that the Pt/MWCNTs nanocomposites prepared under higher pH exhibited higher electrocatalytic activity for methanol oxidation. This demonstration may pave a new way to prepare homogeneous and well-defined nanoparticle catalysts for fuel cell application.
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2. Experimental 2.1. Hydrothermal synthesis of Pt/MWCNTs nanocomposites MWCNTs (purity 95%, diameter 20–60 nm) purchased from Shenzhen Nanotech Port. Co., Ltd (Shenzhen, China) were used as received. Before the deposition of Pt nanoparticle, MWCNTs were functionalized with –C O, –COO and –C–OH groups by refluxing in a 2.6 M nitric acid solution at about 123 ◦ C for 24 h [21]. Pt nanoparticles supported on MWCNTs were synthesized by the hydrothermal technique. A typical preparation consisted of the following steps: 2.7 mL of an aqueous solution of 0.03862 M H2 PtCl6 and 80 mg of purified MWCNTs were added into 10 mL of glycerol (or ethylene glycol (EG)) in a 100 mL vessel. The aqueous NaOH solution was then added, dropwise, to a desired pH value. Then the mixture was ultrasonicated for 30 min to ensure the uniform dispersion of MWCNTs in the solution. Seventy-five milliliters of mixture solution was transferred into a 100 mL Teflon-lined autoclave and maintained at 160 ◦ C for 5 h. Finally, the product was collected through filtration and washed with ethanol and deionized water, and then dried at 373 K for 12 h. The electrocatalysts were labeled as Pt/MWCNTs-1, Pt/MWCNTs-2 and Pt/MWCNTs-3 for pHs 13, 12 and 10, respectively. The Pt/MWCNTs nanocomposite prepared from the ethylene glycol solution of H2 PtCl6 with pHs 13 was referred to as Pt/MWCNTs-4. 2.2. Physical characterization The size of various Pt/MWCNTs nanocomposites was determined on a transmission electron microscope (JEOL-1200EX). For microscopic examinations, the samples were first ultrasonicated in water and then were deposited on Cu grids covered with a continuous carbon film. X-ray photoelectron spectroscopy (XPS) was performed on a VG Escalab 210 electron spectrometer with Mg K␣ radiation. The base pressure of the system was 2 × 10−8 Pa, and the measurements were carried out from 8 × 10−7 to 1 × 10−6 Pa. For each catalyst, a survey spectrum was collected before high-resolutions were recorded. The binding energy of the C 1s of graphite (284.5 eV) was taken as reference. A Philips X’Pert MPD instrument with Cu K␣ radiation was used to obtain the powder X-ray diffraction (XRD) patterns.
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using a CHI660A electrochemical workstation (CH Instruments, Shanghai, China). A three-electrode cell was used with a saturated calomel electrode (SCE) as a reference electrode, a platinum wire as a counter electrode, and a glassy carbon electrode as a working electrode. Before the measurement, all working electrodes were completely activated to a steady state by cyclic voltammetry (20 cycles in 0.5 M H2 SO4 solutions at a scan rate of 100 mVs−1 ). The electrolyte for electrochemical measurements was a solution of 1 M methanol in 0.5 M H2 SO4 . The solution was deaerated with ultrahigh-purity N2 before scanning. The current density was normalized to the geometric area of electrode, unless otherwise specified. 3. Results and discussion 3.1. Characterization of Pt/MWCNTs nanocomposites MWCNTs are considered to be a more attractive candidate due to their extraordinary mechanical characteristics such as high tensile strength coupled with high surface area, high electric conductivity, and thermal conductivity. Pt can be deposited on the MWCNTs by the oxidation of glycerol. In comparison with other reducing reagents such as formic acid [6], ethylene glycol [18], formaldehyde [20], sodium borohydride [22] and hydrazine [23], glycerol is cheaper and environmentally friendly. Here glycerol was used as a reducing reagent and stabilizer. The XRD patterns of Pt/MWCNTs nanocomposites prepared by hydrothermal method are shown in Fig. 1. There is one sharp reflection at about 2θ = 26◦ and three weak reflections at about 2θ = 42.8◦ , 53.8◦ and 77.8◦ that can be indexed to the carbon nanotubes. The diffraction peaks at 2θ of 39.6◦ , 46.2◦ , 67.4◦ and 81.5◦ can be attributed to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) reflection of face-centered cubic (f.c.c.) crystal lattice of platinum. From the full width half maximum of the (2 2 0) peak, the volume averaged particle size was calculated using the Scherrer equation [24]: L=
0.9λk␣1 (B2θ cos θB )
2.3. Electrochemical measurement Electrochemical activity of the synthesized Pt/MWCNTs nanocomposites was evaluated using a glassy carbon electrode (GCE, diameter 3 mm). Prior to coating, GCE was polished sequentially with slurries of 0.3 and 0.05 m alumina to mirror finish. After rinsing with redistilled water, it was cleaned ultrasonically in water for 10 min. To prepare the working electrode, 4 mg of Pt/MWCNTs was dispersed in 1 mL of diluted Nafion solution (0.5 wt%), and 6 L of the suspension was pipetted on to a GCE. The total Pt loading was 68 g cm−2 . Electrochemical measurements were performed
Fig. 1. XRD pattern of Pt/MWCNTs-1 (curve a), Pt/MWCNTs-2 (curve b) and Pt/MWCNTs-3 (curve c) and Pt/MWCNTs-4 (curve d) nanocomposites.
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where L is the average particle size, λk␣1 is the X-ray wave length ˚ for Cu k␣1 radiation), B2θ is the peak broadening, (1.54056 A and θ B is the angle corresponding to the peak maximum. In the obtained samples, the average sizes of Pt nanoparticles are 3, 4.2 and 9.1 nm for pHs 13, 12 and 10, respectively. The results indicate that the size of Pt nanoparticles deposited on MWCNTs is determined by the pH of solution. It was also found that the mean size of Pt nanoparticles is 6.7 nm in the presence of ethylene glycol instead of glycerol. Fig. 2 shows the transmission electron microscopy (TEM) images of various Pt/MWCNTs nanocomposites prepared from the glycerol solution with different pHs. As shown in Fig. 2, the size of Pt particle deposited on MWCNTs surface decreases with the increase of glycerol solution pH, which is in accordance with the XRD results. The average sizes of Pt particles are 2.9, 4.4 and 10.1 nm for pHs 13, 12 and 10, respectively. In the case of pHs 12 and 10, Pt nanoparticles are apparently agglomerated and not well dispersed on MWCNTs, and the particle size ranges from 3 to 15 nm. In the case of pH 13, Pt nanoparticles are hardly agglomerated and well dispersed on the surface of MWCNTs. The above facts indicated that the increase of glycerol solution pH could improve the size uniformity and dis-
persion of Pt nanoparticles on the surface of MWCNTs. Similar phenomenon were also observed by other groups [25–28]. TEM image of Pt/MWCNTs-4 nanocomposites was compared. It is noticed that Pt nanoparticles reduced by ethylene glycol are more easily agglomerated than those reduced by glycerol. The average particle size is 7.1 nm. The chemical composition of the Pt nanoparticles deposited on functionalized MWCNTs was analyzed by XPS. Fig. 3 displays the XPS spectra of various Pt/MWCNTs nanocomposites prepared by the hydrothermal method. The chemical composition of the Pt nanoparticles deposited on functionalized MWCNTs was analyzed by XPS. Fig. 3a displays the narrow scan of Pt (4f) core level of XPS spectra of various Pt/MWCNTs nanocomposites prepared by the hydrothermal method. The spectrum shows a doublet containing a low energy band (4f7/2 ) at about 71.0 eV, and a high energy band (4f5/2 ) centered at 74.5 eV. The area ratio between 4f7/2 and 4f5/2 components is 4/3. Deconvolution of the Pt 4f7/2 region of Pt/MWCNTs-1 shows the presence of two pairs of doublets, which indicates the existence of two different Pt oxidation states (see Fig. 3b). The doublet with binding energy of 70.1 eV was attributed to metallic Pt. Peaks at 72.3 eV could be assigned to Pt(II) as in either PtO
Fig. 2. TEM images of Pt/MWCNTs-1 (a), Pt/MWCNTs-2 (b), Pt/MWCNTs-3 (c) and Pt/MWCNTs-4 (d) nanocomposites.
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Fig. 4. Cyclic voltammograms of Pt/MWCNTs-1 (curve a), Pt/MWCNTs-2 (curve b) and Pt/MWCNTs-3 (curve c) nanocomposites in nitrogen saturated 0.5 M H2 SO4 solutions. Scan rate: 50 mVs−1 .
3.2. Active specific surface area of Pt nanoparticles
Fig. 3. (a) XPS spectra of Pt/MWCNTs-1 (curve a), Pt/MWCNTs-2 (curve b), Pt/MWCNTs-3 (curve c) and Pt/MWCNTs-4 (curve d) nanocomposites. (b) A typical example of XPS Pt 4f peak deconvolution of Pt/MWCNTs-1.
or Pt(OH)2 [7,14]. The percentages of these two species for Pt/MWCNT-1 are calculated from the relative intensity of these peaks are given as 73.9 and 26.1%, respectively (see Fig. 3b). The detailed binding energies and relative intensities of different Pt species on other Pt/MWCNTs catalysts were given in Table 2. It was observed that the content of Pt(0) decreases and the content of Pt(II) increases with increasing the average size of the Pt particles.
The voltammograms of various Pt/MWCNTs nanocomposites prepared under different pHs in 0.5 M H2 SO4 solution are shown in Fig. 4. For the Pt/MWCNTs-3 catalysts, a broad peak with slight current density is observed in the hydrogen adsorption–desorption region. This is probably due to agglomeration and poor dispersion of the Pt nanoparticles in this catalyst. Conversely, for the Pt/MWCNTs-1 and Pt/MWCNTs-2 catalysts, well-defined hydrogen adsorption–desorption peaks with a much lager area are observed in the potential region −0.25 to 0.05 V, demonstrating the higher surface area of above both catalysts. The high surface area is owing to the presence of narrow size and uniform distribution of the Pt nanoparicles as displayed in the TEM images. In general, the real surface area of Pt nanoparticles is an important parameter to determine the catalytic properties of electrocatalysts for methanol electrooxidation since this reaction is surface-sensitive. The areas in m2 g−1 were calculated from the following formula assuming a corresponding value of 0.21 mC cm−2 (calculated from the surface density of 1.3 × 1015 atom per cm2 , a value generally admitted for polycrystalline Pt electrodes) and the Pt loading [29]. m2 QH = AEL −3 (g catalyst) (0.21 × 10 C(g catalyst))
Table 1 Average crystallite size of Pt nanoparticles, active specific area, peak current density and specific current density for methanol oxidation reaction of various Pt/MWCNTs nanocomposites Pt/MWCNTs nanocomposites
d (nm)
QH (mC)
SEL (cm−2 )
AEL (m2 g−1 )
PCD (mA cm−2 )
SCD (mA cm−2 )
Pt/MWCNTs-1 Pt/MWCNTs-2 Pt/MWCNTs-3 Pt/MWCNTs-4
3.0 4.2 9.1 6.7
1.28 1.14 0.30 0.58
4.6 4.1 1.1 2.1
95.8 85.4 22.3 43.8
43 33 4 21
0.66 0.57 0.26 0.71
d: the average size of Pt nanoparticles was obtained by XRD analysis; QH : the amount of charges exchanged during the electroadsorption of hydrogen on Pt nanoparticles; SEL : the specific surface area of Pt nanoparticles obtained electrochemically; AEL : the specific surface area of Pt nanoparticles obtained electrochemically; PCD: peak current density at 0.64 V for methanol oxidation reaction which was normalized to the geometric area of electrode; SCD: specific current density at 0.64 V for methanol oxidation reaction in terms of the real surface area of electrode.
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Electrochemical surface areas of various Pt/MWCNTs nanocomposites are listed in Table 1. The specific surface area of Pt nanoparticles are 95.3, 85.4 and 22.3 m2 g−1 for pHs 13, 12 and 10, respectively. It shows that the real surface area of Pt nanoparticles increases with the increase of synthesis solution pH. This is not surprising in view of the decrease of the metallic particle size with the pH increasing. 3.3. Electrocatalytic activity of Pt/MWCNTs nanocomposites prepared under different pHs Cyclic voltammetry is a valuable and convenient tool for studying methanol oxidation catalysts. The voltammograms of methanol oxidation over Pt/MWCNTs nanocomposites prepared from the glycerol solution with different pHs were very similar to that of the Pt/C catalysts (see Fig. 5). In the forward scan, the current of methanol oxidation becomes apparent as the potential rises about 0.35 V, and methanol oxidation produced a prominent symmetric anodic peak around 0.64 V. As shown in Table 1, the specific catalytic activity of the Pt/MWCNTs nanocomposite for the methanol oxidation follows the order Pt/MWCNTs-1 > Pt/MWCNTs-2 > Pt/MWCNTs-3. It is indicated that the electrocatalytic activity of Pt/MWCNTs catalysts for the methanol oxidation reaction depends strongly on the particle size of dispersed Pt nanoparticles. In the reverse scan, an anodic peak current density was detected at around 0.42 V. It could be attributed to the removal of the incompletely oxidized carbonaceous species formed in the forward scan. The residual carbon species may be oxidized according to the following reaction: PtOHad + Pt C O → CO2 + 2Pt + H+ + e− Hence the ratio of the forward anodic peak current density (If ) to the reverse anodic peak current density (Ib ), If /Ib , can be used to describe the catalyst tolerance to carbonaceous species accumulation. Low If /Ib ratio indicates poor oxidation of
Fig. 5. Cyclic voltammograms of methanol electro-oxidation over the Pt/MWCNTs-1 (curve a), Pt/MWCNTs-2 (curve b) and Pt/MWCNTs-3 (curve c) nanocomposites in 1 M CH3 OH/0.5 M H2 SO4 electrolyte at 50 mVs−1 at room temperature.
methanol to carbon dioxide during the anodic scan and excessive accumulation of carbonaceous residues on the catalyst surface. High If /Ib ratio shows the converse case. Experimentally, the ratios were 1 and 1.1 for Pt/MWCNTs-1 and Pt/MWCNTs-2, respectively. Such a high value indicates that a large amount of intermediate carbonaceous species was oxidized to carbon dioxide in the forward scan. For comparison, the ratio 0.87 was reported with a nanosized Pt on XC-72 synthesized by a microwave-assisted polyol process [6]. These experimental observations highlighted the high activity for methanol oxidation of Pt/MWCNTs nanocomposites prepared by this hydrothermal method. However, the Pt/MWCNTs nanocomposites prepared under pH 10 exhibits slight catalytic activity toward methanol electro-oxidation. It is also found that Pt/MWCNTs nanocomposites prepared under pH < 10 exhibits little catalytic activity (data not shown). It shows that the synthesis solution pH plays an important role in the size of Pt nanoparticles, which is essential for the electroactivity toward methanol oxidation. Fig. 6 shows the change of polarization current densities with time of methanol electro-oxidation on various Pt/MWCNTs catalysts at 0.4 V versus SCE. The current density for the Pt/MWCNTs nanocomposites decayed with time and reached an apparent steady state within 300 s. The polarization current density of Pt/MWCNTs at all corresponding potentials also follows the order Pt/MWCNTs-1 > Pt/MWCNTs-2 > Pt/MWCNTs-3, which is in accordance with the cyclic voltammetry result. The above results clearly show that the specific activity enhanced with the decrease of size of Pt nanoparticles. But there have been contradictory conclusions regarding the influence of size and surface structure of these nanoparticles on the electrocatalytic activity. Some investigators have found that the specific activity with a decrease in platinum particle size, but others have observed different results [30–32]. Stonehart and Watanabe have insisted that the crystallite size effect is not truly dependent on the dimensions of the platinum crystallites but is dependent on the intercrystallites distance [33,34]. According to their hypoth-
Fig. 6. Polarization current densities versus time plots for the electro-oxidation of methanol on Pt/MWCNTs-1 (curve a), Pt/MWCNTs-2 (curve b) and Pt/MWCNTs-3 (curve c) in 1 M CH3 OH/0.5 M H2 SO4 electrolytes at 0.4 V (versus SCE) at room temperature.
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Table 2 Binding energies and relative intensities of different platinum species as observed from Pt (4f7/2 ) XPS of various catalysts Sample
d (nm)
Pt 4f 7/2 peak Position (eV)
Species
Pt 4f7/2 binding energy (eV)
Relative intensity (%)
Pt/MWCNTs-1
3.0
71.03
Pt(0) Pt(II)
70.98 72.30
73.9 26.1
Pt/MWCNTs-2
4.2
70.98
Pt(0) Pt(II)
70.90 71.85
58.5 41.5
Pt/MWCNTs-3
9.1
70.62
Pt(0) Pt(II)
70.61 71.47
37.1 62.9
Pt/MWCNTs-4
6.7
70.87
Pt(0) Pt(II)
70.76 71.79
58.4 41.6
esis, therefore, the specific activity will increase with a decrease in the size of platinum particles if the crystallite separation is shorter than ca. 18 nm. On the other hand, many other investigators have found no such result for the reaction but instead they have found that the specific activity steeply decreased with the decreasing size of platinum particles [35–37]. The essence of size effect of Pt nanoparticles on methanol electro-oxidation is not very clear and needs further investigation. It is generally accepted that the crucial and rate limiting step in methanol electro-oxidation reaction is the oxidation of a methanol adsorbate by an adsorbed hydroxy species or activated water group. When it is assumed that –COH is the methanol adsorbate, the rate-determining step is Pt–COH + Pt–OH → 2Pt + CO2 + 2H+ + 2e−
(1)
The rate of reaction (1) depends on coverage with θ C–OH and θ OH . Since θ C–OH + θ OH < 1, it implies that a high θ OH or θ C–OH will decrease the number of Pt sites that are available for the formation of an adsorbed methanol or hydroxy species, respectively, and the reaction rate will consequently be low. Hence it is obvious that the efficiency of the Pt catalyst would be improved if a well balanced co-adsorption of methanol and water could be realized at low potential. It was observed that the shift to higher potentials for the Pt oxide reduction with increasing particle size in Fig. 4. Also, the onset of oxidation is shifted to higher potentials with increasing particle size. This is indicative of a lower Pt–O affinity for larger Pt nanoparticles and implies that the ability to activate water, i.e. to break H–OH bonding with the formation of a Pt–OH species, proceed at a lower potential for small particles. Hence, it was assumed that the smaller sized catalysts contain more Pt–OH groups at a given potential than larger ones. In view of reaction (1), it was expected that an increase in specific activity for the methanol oxidation with a decrease in particle size. On the other hand, the intensity of Pt oxides enhanced with the increase of the particle size, as shown in Table 2. It was implied that the smaller particles leave fewer Pt sites for methanol adsorbates. Owning to the smaller amount of the methanol adsorbates, the specific activity of the large Pt particles is low. The above two reasons could imply that Pt/MWCNTs nanocomposites with smaller diameter Pt nanoparticles exhibited better intrinsic activity.
3.4. Electro-oxidation of methanol over Pt/MWCNTs-1 nanocomposites The effect of the scan rate on the electro-oxidation of methanol over Pt/MWCNTs-1 nanocomposites is shown in Fig. 7. In the range of 20–200 mVs−1 , the peak current density increases linearly with the square root of the scan rates. It shows that the methanol electro-oxidation process on the Pt/MWCNTs1 nanocomposites is controlled by the diffusion of methanol to the electrode surface. Additionally, the peak potential (the forward scan) increases with the increase of scan rate. It is indicated that the oxidation of methanol is an irreversible electrode process. Fig. 8 shows the effect of methanol concentration on the oxidation reaction for Pt/MWCNTs-1 nanocomposites in 0.5 M H2 SO4 at room temperature. Anodic peak current density increased with the concentration of methanol in the region of 0.1–4 M. However, a saturation of the current response was observed at methanol concentration of 6 M. The oxidation of methanol produces CO and possibly other adsorbed intermediates, which can be oxidized with the participation of oxygenated species formed on the surface. The latter process is inhibited by the adsorption of methanol on the surface, which increases
Fig. 7. Cyclic voltammograms of methanol electro-oxidation on the Pt/MWCNTs-1 nanocomposites in 1 M CH3 OH/0.5 M H2 SO4 electrolyte at various scan rates at room temperature. Scan rate (from inner to outer): 20, 40, 60, 80, 100, 150 and 200 mVs−1 .
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Fig. 8. Linear voltammograms of methanol oxidation on Pt/MWCNTs-1 nanocomposites at 50 mVs−1 in (a) 0.1 M, (b) 0.5 M, (c) 1 M, (d) 2 M, (e) 3 M, (f) 4 M CH3 OH/0.5 M H2 SO4 electrolyte.
Fig. 10. Cyclic voltammograms of Pt/MWCNTs nanocomposites prepared from the ethylene glycol solutions of H2 PtCl6 with pH 13 in the presence of MWCNTs in (a) nitrogen saturated 0.5 M H2 SO4 and (b) nitrogen saturated 0.5 M H2 SO4 + 1 M CH3 OH. Scan rate: 50 mVs−1 .
with the increase in methanol concentration. Cyclic voltammetry also detected a slow but gradual current density decrease with cycling, which is often attributed to the accumulation of strong chemisorbed species on the catalyst surface. The effect of the potential scan limit on the peak current density is shown in Fig. 9. The forward scan peak current density increases with increasing the anodic limit in the forward scan. The reverse anodic peak potential shifted negatively with increasing the anodic limit in the forward scan, while the peak current density decreases. This behavior indicates that the reverse anodic peak current density is primarily associated with residual carbon species on the surface of the Pt/MWCNTs-1 electrode.
and 0.5 M H2 SO4 + 1 M CH3 OH solutions are presented in Fig. 10. For this catalyst, the specific surface area is 43.8 m2 g−1 . The peak current density at 0.64 V was 21 mA cm−2 , which is one half of the value of 43 mA cm−2 obtained for the reaction on Pt/MWCNTs-1. It is indicated that Pt/MWCNTs-1 possesses better electrocatalytic activity toward methanol oxidation compared to Pt/MWCNTs-4. At high temperature, glycerol exhibits strong reducing ability, especially in alkaline condition. H2 PtCl6 could be readily reduced to Pt nanoparticle by glycerol at 160 ◦ C. The possible reaction pathway could be illustrated by following equation:
3.5. Comparison of electrocatalytic activity of Pt/MWCNTs nanocomposites synthesized by different reducing agents The cyclic voltammograms of Pt/MWCNTs nanocomposites prepared by ethylene glycol reduction method in 0.5 M H2 SO4
Fig. 9. Cyclic voltammograms of room temperature methanol electro-oxidation over Pt/MWCNTs-1 nanocomposites at 50 mVs−1 in 1 M CH3 OH/0.5 M H2 SO4 electrolyte for different forward potential scan limits.
C3 H5 (OH)3 + 3PtCl6 2− + 16OH− → 18Cl− + CO3 2− + C2 O4 2− + 12H2 O + Pt For this reaction pathway to take place, the –OH groups of glycerol interact with Pt-ion sites, resulting in the oxidation of glycerol to oxalate and carbonate in alkaline media. Under alkaline condition, small and uniform Pt nanoparticles deposited on MWCNTs could be prepared by above hydrothermal method. There are two reasons for it. One is that functionalized MWCNTs could interrupt metal particles agglomeration due to their anchorage on MWCNTs surface. It is known that nitric acid treatment could introduce carboxylic acid groups of the open ends and defect sites of the MWCNTs. At strong alkaline condition, a high intensity of carboxylate on the surface of MWCNTs was reached. These surface groups could act as anchoring sites and/or nucleating sites for nanoclusters in the reduction step of Pt precursor, and therefore prevent Pt particles from agglomerating. On the other hand, oxalate is believed to be act as a stabilizer for the Pt colloids, possibly forming chelate-type complexes via its carboxyl groups [26,38]. Such interactions between the Pt colloids and acid form (oxalic acid) are smaller. Oxalic acid is thus believed to be poor stabilizer for noble metal nanoparticle. Therefore, the pH of the synthesis solution is expected to greatly influence the stability and size of the metal nanoparticles. On the basis of the above, Pt nanoparticles prepared with low pH should
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be unstable and easily agglomerated, thus have large size and wide size distribution. With the increase of the synthesis solution pH, the Pt nanoparticles would become smaller and more uniform, as shown in Fig. 1a–c. In comparison with EG, smaller and more uniform Pt nanoparticles supported on MWCNTs were reduced by glycerol in hydrothermal environment. It could be due to the fact that oxalate is a good stabilizer for Pt colloid and glycerol has more hydroxyl groups that could prevent Pt colloids from agglomerating. 4. Conclusions In summary, Pt nanoparticles deposited on MWCNTs were prepared by a facile hydrothermal method. Glycerol was utilized as a reducing reagent and stabilizer. The dispersion and size of Pt nanoparticles in Pt/MWCNTs nanocomposites are remarkably affected by the synthesis solution pH. It is clarified that Pt/MWCNTs nanocomposites with smaller size of Pt nanoparticles exhibited higher electrocatalytic activity toward methanol electro-oxidation by cyclic voltammetry and chronoamperometry. The essence of the size-dependent activities of Pt/MWCNTs toward methanol electro-oxidation was also discussed. Acknowledgements This work was supported by the National Natural Science Foundation of China (Project number, 90210027) and the China Nation Key Basic Research Special Funds (Project numbers, 2003CB214500, and 2007CB613305). References [1] M. Winter, R.J. Brodd, Chem. Rev. 104 (2004) 4245. [2] B.D. McNicol, D.A.J. Rand, K.R. Williams, J. Power Sources 83 (1999) 15. [3] K.Y. Chan, J. Ding, J. Ren, S. Cheng, K.Y. Tsang, J. Mater. Chem. 14 (2004) 505. [4] F. Bensebaa, A.A. Farah, D. Wang, C. Bock, X. Du, J. Kung, Y. Le Page, J. Phys. Chem. B 109 (2005) 15339. [5] S. Liao, K.A. Holmes, H. Tsaprailis, V.I. Birss, J. Am. Chem. Soc. 128 (2006) 3504. [6] Z. Liu, X.Y. Ling, X. Su, J.Y. Lee, J. Phys. Chem. B 108 (2004) 8234. [7] J. Prabhuram, X. Wang, C.L. Hui, I.M. Hsing, J. Phys. Chem. B 107 (2003) 11057.
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