PtâSn Nanoparticles Decorated Carbon Nanotubes as

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Pt-Sn Nanoparticles Decorated Carbon Nanotubes as Electrocatalysts with Enhanced Catalytic Activity Chien-Te Hsieh, Yan-Shuo Chang, and Ken-Ming Yin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp402609s • Publication Date (Web): 24 Jun 2013 Downloaded from http://pubs.acs.org on June 25, 2013

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The Journal of Physical Chemistry

Pt−Sn Nanoparticles Decorated Carbon Nanotubes as Electrocatalysts with Enhanced Catalytic Activity

Chien-Te Hsieh*, Yan-Shuo Chang, Ken-Ming Yin Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze University, Taoyuan 32003, Taiwan

(TEL): 886-3-4638800 ext. 2577 (FAX): 886-3-4559373 (E-MAIL): [email protected]

Prepared for submission to The Journal of Physical Chemistry: C March 15, 2013 Revised June 10, 2013

*To whom correspondence should be addressed.

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ABSTRACT The catalytic activities of Pt−Sn alloy nanoparticles with different Pt:Sn atomic ratios, prepared by a pulse microwave-assisted polyol (MP) method, in the oxygen reduction reaction and oxidation of methanol and formic acid have been examined by using cyclic voltammetry. The pulse MP approach enables the formation of Pt−Sn alloy nanoparticles with well-defined atomic ratios over the surface of carbon nanotubes. The as-prepared Pt−Sn nanoparticles display a homogeneous dispersion with a narrow crystalline size in the range of 2.72−3.66 nm. An appropriate amount of Sn dopants (25 at%) facilitates not only the catalytic activity but also the long-term anti-poisoning ability, as compared with pure Pt catalyst. The improved performance of Pt−Sn alloy catalyst is attributed to the bi-functional mechanism of bimetallic catalysts; that is, CO adsorption mainly occurs on Pt sites, while OH formation would take place preferentially on the Sn sites. Thus, the introduction of Sn offers one pathway to strip CO from the Pt–CO sites, thereby raising the CO tolerance. Without any treatment, the pulse MP synthesis emerges as a feasible method to prepare Pt–Sn catalysts with excellent catalytic activity and long-term durability for fuel cell applications.

Keywords: Pt–Sn alloys; Catalytic activity; Formic acid oxidation; Methanol

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oxidation; Fuel cells

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1. Introduction Fuel cells have been regarded as green technology for future energy due to their high energy efficiency, low pollutant emission, and clean utilization.1-3 Nobel metallic catalysts, such as Pt supported on highly porous carbons, are typically used as electrocatalysts for usage in fuel cells. To date, one of the major issues concerning small molecule (e.g., methanol and formic acid) fuel cells still exists; that is, the poisoning of the electrocatalysts by CO formed during the incomplete oxidation of organic fuels.4 To raise the performance of Pt-based catalysts, several types of binary Pt−M catalysts such as M= Zn,5,6 Fe,7-9 Co,7-14 Ni,7,8,15 Sn,16,17 and Ag,18 have been investigated as a replacement for pure Pt catalyst in various applications of fuel cells. The improved catalytic activity on the Pt−M catalysts can be attributed to the so-called bifunctional mechanism in methanol and formic acid electro-oxidation.4 Additionally, these pioneering studies regarding Pt−M catalyst5-18 also confirmed that the presence of secondary element could contribute to a decreasing cost to the Pt-based catalysts. Among the Pt−M catalysts, the Pt−Sn pair has been demonstrated to replace the Pt−Ru pair owing to its high catalytic activity and good capability of CO tolerance.19 However, schematic studies of Pt−Sn catalysts are still scarce due to the difficulty in the synthesis of the binary nanoparticles of controlled compositions. Earlier attempts to deposit Pt nanoparticles on different types of carbon supports,

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including carbon nanotubes (CNTs) and carbon black, have been performed using chemical-wet approach using ethylene glycol as the reduction agent.12,20 The recent investigation in the optimization of Pt catalysts and high accessible surface area carbon supports has been the replacement of conventional carbon black by CNTs.6,21 This can be ascribed to the fact that CNTs display the benefits including high surface area, good electrical conductivity, and corrosion resistance to acid electrolyte. Recently, microwave-assisted polyol (MP) method becomes an emerging approach to deposit Pt nanocatalysts on CNTs,6,22 owing to its energy efficiency, speed, uniformity, and simplicity in execution. During the MP process, the reduction reaction is activated with the aid of continuous irradiation of microwaves. Thus, a rapid heating can take place under microwave irradiation.23 Recently, Wang et al. adopted MP method to immobilize Pt−Sn bimetallic catalysts on N-doped CNTs for alcohol oxidation, showing superior electrocatalytic performance.24 Herein we propose one-step pulse MP route to precisely control the atomic compositions of binary Pt−Sn nanoparticles over the surface of CNTs. In the pulse MP method, the ratio of microwave on time (ton) to off time (toff) contributes to uniform temperature profile of the reaction mixture,25 inducing well dispersed Pt−Sn nanoparticles attached to the CNTs. Within the above scope, this present work aims at the fast deposition of Pt−Sn/CNT electrocatalysts using one-stage pulse MP method. The total deposition

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period of the MP method only takes ca. 10 min, and the resulting Pt−Sn catalysts show the uniform dispersion over the surface of CNTs. A comparison of the Pt−Sn catalysts with different Pt:Sn atomic ratios was made to characterize catalytic activity and stability of as-deposited Pt−Sn catalysts, based on cyclic votammetry (CV) measurements in sulfuric acid, methanol, and formic acid electrolytes. The merit of this study is to shed some lights on the influence of Sn atomic ratio on the catalytic activity toward oxygen reduction reaction and oxidation of methanol and formic acid.

2.

Experimental

2.1. Pulse MP synthesis of Pt−Sn electrocatalysts The continuous MP synthesis of growing Pt−Sn electrocatalysts has been reported in our previous paper.17 The pulse MP method could be briefly described as follows. First of all, multi-walled CNTs used here were prepared by a catalytic chemical vapor deposition method, using ethylene and Ni particle as the carbon precursor and catalyst, respectively. The CNTs were chemically oxidized by immersing them in 3 N nitric acid at 90°C for 3 hr, allowing the implantation of surface oxides such as carboxyl (−COOH), hydroxyl (−OH), and carbonyl (−CO) on both ends and defects of CNTs. The treated CNT samples were then dehydrated at 105°C in vacuum oven overnight. Afterward, the oxidized CNTs (ca. 100 mg) were 6


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placed into in Pt ionic solution, consisting of 2 ml of 0.04 M PtCl4 and 30 ml of ethylene glycol. The initial pH of the CNT slurry was approximately 2.3, and the CNT slurries were adjusted to pH= 11 by adding sufficient amount of 0.04 M KOH. The CNT slurry was put into the center of a household microwave oven and then heated under microwave irradiation. Meanwhile, the maximal power of the microwave oven was set at 720 W. The ratio of ton to toff and total cycle number was 3:2 and 120 cycles, respectively. The maximal temperature was maintained at ca. 150 °C by using the temperature controller. The resultant Pt/CNT composite was washed with de-ionized water, filtered and dried in a vacuum oven at 90°C overnight. One-stage pulse MP procedure for depositing binary Pt–Sn catalysts on the CNTs was described as follows. The oxidized CNTs were initially impregnated with a Ptand Sn-containing solution in a baker. As for the preparation of Pt50Sn50 catalyst, the ionic solution was composed of 2 ml of 0.04 M PtCl4, 2 ml of 0.04 M SnCl4, and 30 ml of ethylene glycol. Similarly, the CNT slurries were also set at pH= 11 by using 0.04 M KOH additive. The operating conditions for the pulse MP synthesis of Pt–Sn catalysts were identical with the synthesis of Pt/CNT composite. To examine the effect of Pt:Sn atomic compositions, three ionic solutions were prepared to synthesize four types of Pt−Sn nanoparticles, i.e., Pt75Sn25, Pt50Sn50, and Pt25Sn75. A drop-coating method was used to fabricate Pt−Sn/CNT layer onto carbon paper (CP), forming a

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composite electrode. The preparation for the catalyst ink was described as follows. The Pt−Sn/CNT catalyst (25 mg) was first mixed with isopropyl alcohol (12.5 ml) and 5.0 wt% Nafion® solution (0.28 ml) in a scintillation vial, and the suspension was dispersed in an ultra-sonicated bath for 1 hr, ensuring a homogeneous dispersion of catalyst ink. A CP (SGL 10BC) with an area of 5 × 5 cm2 was used as the substrate. The well-dispersed catalyst ink was carefully dropped and coated over the CP substrate to obtain a GDE with Pt−Sn/CNT loading of 0.8–1.0 mg cm-2.

2.2. Characterization of the Pt−Sn electrocatalysts The structural observation of as-prepared Pt−Sn/CNT composites was investigated using field-emission scanning electron microscope (FE-SEM, JEOL 2010F) and transmission electron microscope (TEM, JEOL, JEM-2100). Energy diffraction spectroscopy (EDS) analysis was adopted to examine the atomic ratio of Pt to Sn in the bimetallic alloy. A thermogravimetric analyzer (TGA, Perkin Elmer TA7) was adopted to analyze the amount of catalysts deposited on the carbon support. The TGA analysis was carried out under an air atmosphere at a heating rate of 10 °C min-1 with the temperature range of 30−1000°C. The crystalline structure of the Pt−Sn catalysts was determined by X-ray diffraction (XRD) with Cu-Kα radiation, using an automated X-ray diffractometer (Shimazu Labx XRD-6000). The mean crystalline

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size of the Pt−Sn catalysts could be calculated through the XRD patterns according to Scherrer’s formula d XRD =

0.9λ β1 / 2 cos θ

(1)

where dXRD is the average crystalline size (nm), λ the wavelength of X-ray (0.15406 nm), θ is the angle at the peak maximum, and β1/2 is the width (radians) of the peak at half height.

2.3. Electrochemical activity of Pt−Sn electrocatalysts To inspect the electrochemical activity of Pt−Sn catalysts, the CV measurements were conducted in three types of electrolytes including sulfuric acid, methanol, and formic acid. The area (2 × 1 cm2) for the working electrode (i.e., Pt−Sn/CNT composites) was the geometric area exposed to electrolyte. Herein Pt wire and Ag/AgCl electrode served as counter and reference electrodes, respectively. In the first part, the CV measurement of Pt−Sn/CNT electrodes was carried out within in the potential range from -0.2 V to 0.8 V vs. Ag/AgCl at ambient temperature using 1 M H2SO4 as the electrolyte solution. The working electrodes were constructed by pressing the Pt−Sn/CNT composites onto stainless steel foil, which served as current collector. The potential scan rate and scan number were set at 30 mV s-1 and 1000 cycles, examining the activity and durability of the binary Pt−Sn catalysts in acid

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electrolyte. One crucial index, electrochemically surface area (ECSA) value, based on the specific charge transfer, contributed from the Pt-based catalysts (QH), could be determined by: ECSA =

QH 210 ⋅ m

(2)

where m is the catalyst loading in units of g cm-2, and the factor 210, in units of μC cm-2, was obtained from electrical charge associated with monolayer adsorption of hydrogen on Pt.26 In the second part, the electrooxidation of methanol on the Pt−Sn/CNT electrodes were also performed in the same three-electrode configuration at ambient temperature using 0.5 M H2SO4 + 0.5 M CH3OH as the electrolyte. The methanol oxidation of the catalyst electrodes was carried out in the potential range of 0–1.0 V vs. Ag/AgCl with a sweep rate of 30 mV s-1. The third part, electrooxidation of formic acid on the electrodes was performed at ambient temperature using 0.5 M H2SO4 + 0.5 M HCOOH as the electrolyte solution. The formic acid oxidation of the catalyst electrodes was also carried out in the potential range of 0–1.0 V vs. Ag/AgCl with a sweep rate of 30 mV s-1. All electrochemical measurements in this work were performed under N2 atmosphere at ambient temperature.

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3.

Results and Discussion

3.1. Characterization of Pt−Sn/CNT composites Top-view FE-SEM and bright-field TEM micrographs of the resultant Pt−Sn/CNT composite, prepared by the pulse MP method, are depicted in Figure 1(a) and 1(b), respectively. As observed form these views, the CNTs show a coiled shape, an average diameter of 20−50 nm, a length of several micrometers. There are a large number of white dots (i.e., Pt75Sn25 nanoparticles), attached to the surface of CNTs. The average size of Pt75Sn25 nanoparticles is approximately 2−5 nm. The selected-area electron diffraction focusing on the Pt75Sn25 nanoparticles is illustrated in Figure 1(c). The appearance of bright diffraction spots along with diffraction rings reflects that the Pt75Sn25 particles consist of nanocrystallites. As shown in Figure 1(d), the Pt−Sn nanoparticles basically display a homogeneous dispersion with narrow particle size distribution, ranged from 2.2 to 4.2 nm. Figure 2 shows the weight loss curves of different Pt−Sn catalysts within the entire temperature range of 30−1000°C, determined from the TGA analysis. The curves clearly show an obvious weight loss, which starts from 400 to 750°C. For all samples, two distinct steps (i.e., 400−600°C and 600−750°C) appear, presumably resulting from gaseous oxidation of amorphous carbon (i.e., CP substrate) and multi-walled CNTs in air atmosphere, respectively.27,28 Herein the residual weight

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percentages, assigned to the weight of metallic Pt−Sn catalysts, are used to calculate the catalyst loading onto the carbon supports. The calculated results for all Pt−Sn catalysts are collected and listed in Table 1. The surface densities are found to fall into the range of 0.21−0.26 mg cm-2. The atomic ratios of Pt to Sn, determined from the EDS analysis of the Pt−Sn/CNT composites, are also listed in Table 1. This table reveals that these Pt−Sn particles show their own atomic ratios of Pt:Sn, approximately close to their molar ratios of Pt to Sn in the ionic solutions. On the basis of the results, the pulse MP technique displays a potential feasibility to prepare binary Pt−Sn catalysts with well-defined atomic ratios. Typical XRD patterns of Pt−Sn/CNT composites with different Pt:Sn atomic ratios are illustrated in Figure 3. It can be observed from the XRD patterns that a diffraction peak takes place at ca. 2θ = 26°, which is attributed to the graphitic structure of CNTs. Pure Pt100 displays a typically face-centered cubic (fcc) structure, showing the representative peaks of the fcc structure at 2θ ≈ 40° and 47° for the (111) and (200) planes, respectively.23 As compared with the other Pt−Sn/CNT composites, there are no additional reflections after the addition of Sn elements, implying the presence of Pt−Sn alloys onto the surface of CNTs (i.e., not core-shell structures). The formation of pure Pt nanoparticles with fcc structure was confirmed, i.e., the lattice constant of 0.391 nm agrees well with the crystallographic data.29 The lattice

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constants of bimetallic Pt−Sn catalysts are calculated to be 0.389 nm (Pt75Sn25), 0.388 nm (Pt50Sn50), and 0.385 nm (Pt25Sn75). The introduction of Sn atoms induces the deviation of lattice constants (0.51−1.53%), originated from the presence of Sn dopants into the fcc crystals. This result proves that the pulse MP synthesis is capable of forming Pt−Sn alloys with high alloying degree. This result is similar to our previous study;17 however, the pulse MP route used here takes advantage of without using any further heat treatments, such as calcination. The growth of binary nuclei presumably takes place simultaneously in the ionic solution under the pulse microwave irradiation, thus inducing uniformly atomic arrangement in the Pt−Sn alloys. According to the calculations of Eq. (1), the dXRD values for all Pt−Sn alloys are also listed in Table 1. The dXRD values show a slight increase from 2.72 to 3.66 nm, possibly resulting from the incorporation of Sn in the bimetallic alloy. The average size of Pt−Sn nanocatalysts is in good agreement with the TEM observation.

3.2. ECSA of Pt−Sn/CNT electrodes Figure 4 shows cycle voltammograms of Pt100/CNT and Pt75Sn25/CNT electrodes in 1 M H2SO4 at different sweep rates of 1, 5, 10, and 30 mV s-1 under N2 atmosphere. Herein the CV scans start from an open-circuit potential and sweep within the entire potential region between -0.2 and 0.8 V vs. Ag/AgCl. Both catalyst electrodes display

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well-defined hydrogen adsorption/desorption peaks in the potential region of -0.2–0 V vs. Ag/AgCl,30,31 i.e., a redox reaction on Pt-based catalysts: Pt + H+ + e– ↔ Pt–Hads. As observed from the CV profiles, an increase in the peak current with the sweep rate reflects well-dispersion of Pt nanocatalysts over CNTs, leading to fast kinetics of redox reaction. The ECSA value can be considered as an index to evaluate the catalytic activity of Pt−Sn alloys onto the CNTs. The ECSA values at the first CV cycle with 30 mV s-1 are estimated and listed in Table 1, based on Eq. (2). The Pt75Sn25 catalyst exhibits the highest ECSA value (i.e., 50.7 m2 g-1) among these electrodes. This result reveals that the ratio of Pt:Sn strongly affects the catalytic activity in terms of the number of active sites, with the optimal ratio in this series of materials revealed to be Pt75Sn25. To examine the catalytic stability, the Pt−Sn/CNT electrodes were repeatedly cycled at a scan rate of 30 mV s-1, as shown in Figure 5. After 1000 CV cycles, all Pt−Sn catalysts still maintain stable activity, and the ECSA values have an order as follows: Pt75Sn25 (56.1 m2 g-1) > Pt100 (46.5 m2 g-1) > Pt50Sn50 (26.3 m2 g-1) > Pt25Sn75 (14.6 m2 g-1). It is generally recognized that pure Sn particles tend to easily leach from the Pt surface in acidic electrolyte. However, the Pt−Sn catalysts display a strong resistance to acid corrosion due to their high alloying degree. The result of stability test for all catalysts releases one crucial message: the interface between Pt−Sn alloys

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and CNT support shows an anti-corrosion ability in electrolysis reaction, demonstrated by one previous study.32 Thus, such robust architecture of Pt−Sn/CNT electrodes by the pulse MP method prevents the catalyst loss or crystal mergence in acid electrolyte. It is worth noting that the Pt75Sn25 catalyst exhibits not only the enhanced catalytic activity but also the excellent cycleability. It is rational to assume that the higher ECSA value may signify the better catalyst that possesses more sites available for electrocatalytic reaction. The introduction of Sn element with appropriate content (i.e., 25 at%) effectively intensify the number of active sites for hydrogen adsorption. This is possibly originated from the Sn dopants in the Pt-rich crystals, inducing an increase in amount of point defects that improve the catalytic activity. To inspect the activity for oxidation reduction reaction (ORR), a linear sweep voltammetry (LSV) was conducted in 0.5 M H2SO4 at a sweep rate of 30 mV s-1 with an O2 gas flow of 100 ml min-1. Prior to the LSV measurement, high-purity oxygen was introduced into the three-electrode system for 0.5 hr. The LSV profiles of Pt−Sn catalysts are depicted in Figure 6. Apparently, Pt75Sn25 delivers the highest ORR activity, followed by Pt100, Pt50Sn50, and Pt25Sn75, agreeing with the sequence of ECSA values. The reaction mechanism of ORR on the Pt surface can be represented as follows:33

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Pt + O2 → Pt−O2

(R1)

Pt−O2 + H+ + e− → Pt−HO2(ads)

(R2)

Pt−HO2(ads) + Pt → Pt−OH + Pt−O

(R3)

Pt−OH + Pt−O + 3H+ + 3e− → 2Pt + 2H2O

(R4)

The improved ORR activity on Pt75Sn25 catalyst is presumably attributed to the explanation. Sn dopants facilitate the stripping of oxygenate species (e.g., −OH) from the surface of Pt75Sn25 alloy particles, referring to the fact that Sn (~1.96) shows an electronegativity lower than Pt (~2.28). This phenomenon is also observed by the other co-workers.15 This change in the electronic density of Pt surface, caused by the Sn dopants, would induce low adsorption energy of oxygenated species onto the alloy particles, thus weakening the Pt–O bond strength. However, excessive Sn amount (e.g., Pt50Sn50 and Pt25Sn75) is unfavorable for the ORR activity, originated from the decrease in Pt site number available for the Pt–O bonding. On the basis of the experimental results, an optimal Pt:Sn atomic ratio in the alloy catalysts is needed to improve the catalytic activity.

3.3. Methanol electrooxidation on Pt−Sn/CNT electrodes Figure 7 shows cycle voltammograms of Pt−Sn/CNT electrodes in 0.5 M H2SO4 + 0.5 M CH3OH at a sweep rate 30 mV s-1. The CV scans start from open-circuit

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potential and sweep within the potential region between 0 and 1 V vs. Ag/AgCl. It can be seen that two typical oxidation peaks occur for both Pt−Sn/CNT electrodes. The onset and forward potentials for all alloy catalysts are listed in Table 2. Compared with pure Pt catalysts, the addition of Sn metals leads to a lower anodic peak potential, resulting in the advance of methanol oxidation. Generally, the onset potential can be taken into account as an indicator in determining the catalytic activity for methanol oxidation.3,4 As shown in Table 2, the sequence of onset potential reveals that an appropriate amount of Sn dopants improves the progress of methanol oxidation at lower potential, implying a better electrocatalysis capability. The forward scan is assigned to methanol oxidation, forming Pt-adsorbed carbonaceous intermediates. The oxidation reactions are illustrated as follows:34 Pt + CH3OH → Pt–CO(ads) + 4H+ + 4e−

(R5)

CH3OH + H2O → CO2 + 6H+ + 6e−

(R6)

This adsorbed CO species (i.e., Pt–CO(ads)) significantly induces the poisoning of Pt catalysts, reducing the activity of Pt-based catalyst. The backward scan is attributed to the additional oxidation of adsorbed carbonaceous species to CO2. Pt–CO(ads) + H2O → Pt + CO2 + 2H+ + 2e−

(R7)

Generally, the ratio of the forward peak current (JF) to the backward peak current (JB) serves as an estimation the ratio of the amount of methanol oxidized to CO2 to CO.

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The magnitudes of JF for all Pt−Sn/CNT electrodes are collected in Table 2, showing the importance of Pt:Sn atomic ratio on the catalytic activity toward methanol oxidation. The Pt75Sn25 catalyst displays an enhancement of 28% as compared with Pt100 catalyst, whereas the other two catalysts possess much low JF values < 1.13 A g-1. Moreover, the JF/JB ratio of Pt75Sn25 catalyst can be achieved as high as 1.68, higher than that of Pt100 catalyst (i.e., JF/JB: 1.50). After 200 cycles, the JF/JB ratio of Pt75Sn25 catalyst shows a gradual increase to 2.15, while the Pt100 catalyst has a low JF/JB ratio of 0.8. Accordingly, the Pt75Sn25 catalyst delivers the best electrocatalytic activity and the superior anti-poisoning performance in methanol oxidation. To confirm the long-term stability, a chronoamperometry test at a constant potential of 0.45 V vs. Ag/AgCl was conducted in electrolyte 0.5 M H2SO4 + 0.5 M CH3OH, as shown in Figure 8. Initially, all potentiostatic currents display a rapid decrease, originated from the formation of intermediate species such as Pt–CO(ads), Pt–CH3OH(ads), and Pt–CHO(ads) in the electrooxidation of methanol.35 After that, the decay tends to be gradual and then remains at stable currents. This long-term decay is attributed to surface-adsorbed SO42− anions on the Pt-based catalysts, preventing the progress of methanol oxidation reaction.36 The steady-state polarization curves clearly indicate that the Pt75Sn25 catalyst offers the highest stable current among these catalysts. This enhanced anti-poisoning effect on the Pt75Sn25 catalyst is attributed to

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the bi-functional mechanism;37 that is, CO adsorption mainly occurs on Pt sites, while OH formation would take place preferentially on the Sn sites. Thus, the lateral interaction between CO- and OH-adsorbed species facilitates the enhanced effect of CO oxidation on the Pt75Sn25 catalyst, leading to a high-level of CO tolerance in methanol oxidation.

3.4. Formic acid electrooxidation on Pt−Sn/CNT electrodes Figure 9 depicts cycle voltammograms of Pt−Sn/CNT electrodes in 0.5 M H2SO4 + 0.5 M HCOOH at 30 mV sec-1. The CV profiles of both catalyst electrodes show a similar configuration but different mass current densities. The positive scan consists of two peaks at ca. 0.36 and 0.72 V vs. Ag/AgCl, corresponding to direct and indirect oxidation of formic acid. As to the direct pathway, a dehydrogenation reaction on the Pt sites at low potential can be formulated as follows.38 Pt + HCOOH → Pt–COOH¯ + H+ → Pt + CO2 + 2H+ + 2e−

(R8)

Herein the direct pathway mainly occurs on Pt sites unoccupied by CO at low potentials. The indirect pathway for formic acid oxidation is accompanied with the oxidation of Pt–CO(ads) species that rises from the dissociation of formic acid at high potentials. The reaction of indirect pathway is illustrated as follows.39 Pt + HCOOH → Pt–CO(ads) + H2O → Pt + CO2 + 2H+ + 2e−

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(R9)


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Table 3 collects the mass current density at direct and indirect oxidation peaks (i.e., Jdirect and Jindirect) for all catalyst electrodes. As compared with the other catalysts, the Pt75Sn25 catalyst exhibits the highest Jdirect and Jindirect values, showing the best catalytic activity toward electrooxidation of formic acid. After the potential cycling, the Pt75Sn25 catalyst still possesses the highest mass activity, and its Jdirect/Jindirect ratio shows an increasing trend from 21.0 to 57.1%. The enhanced Jdirect/Jindirect ratio reflects the direct pathway for formic acid oxidation (i.e., (R8)) on the Pt75Sn25 alloy with high CO tolerance. Additionally, one cathodic peak within the potential region between 0.25−0.45 V vs. Ag/AgCl can be observed in the backward scan, as shown in Figure 9. This presence of oxidation peak mainly originates from the regeneration of Pt−CO(ads) or other oxide sites from the dehydration reaction of HCOOH, making these sites available for formic acid oxidation.40,41 As expected, the Pt75Sn25 catalyst displays the highest catalytic activity (~110 A g-1) among these catalysts, indicating the recovery of high coverage of exhausted Pt−CO(ads) sites. To examine long-period anti-poisoning ability in the presence of formic acid, Figure 10 depicts the amperometric curves of Pt−Sn/CNT catalysts in 0.5 M H2SO4 + 0.5 M HCOOH at 0.7 V. The potential selected here is based on the indirect oxidation of formic acid at Pt−CO(ads) sites, followed by the regeneration in the presence of Pt–OH or Sn–OH sites. It can be

20


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found that after the initial stage (< 300 s), the stable current of Pt75Sn25 catalyst is achieved as high as 8.51 A g-1. This result proves the presence of one pathway to strip carbon monoxide from the Pt–CO(ads) sites, thereby raising the CO tolerance. The reaction can be proposed and listed as follows. Sn–OH + Pt–CO(ads) → Sn + Pt + CO2 + H+ + e−

(R10)

Accordingly, the introduction of Sn dopants provides the positive effect on the improvement of catalytic stability in formic acid. On the basis of the experimental results, an appropriate Pt:Sn ratio in Pt−Sn alloy catalysts could facilitate not only superior electrocatalytic activity toward H2 adsorption, methanol and formic acid oxidation but also a long-term anti-poisoning capability.

4.

Conclusions A series of Pt−Sn alloy nanoparticles were deposited onto CNT supports by

using the pulse MP method. The pulse MP route delivered a rapid deposition method to prepare binary Pt−Sn catalysts with well-defined atomic ratio. The as-prepared Pt−Sn nanoparticles showed a homogeneous dispersion with a narrow crystalline size in the range of 2.72−3.66 nm. The appropriate amount of Pt:Sn atomic ratio (=75:25) demonstrates positive effect on the improved catalytic activity through the CV

21



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measurements in sulfuric acid, methanol, and formic acid electrolytes. Electrochemical testing results revealed that the introduction of Sn dopants enhances not only the catalytic activity toward electroxidation of methanol and formic acid but also the long-term anti-poisoning stability, as compared with pure Pt catalyst. The improved performance of Pt−Sn alloy catalyst can be achieved, originated from the bi-functional mechanism of bimetallic catalysts; that is, CO adsorption mainly occurs on Pt sites, while OH formation would take place preferentially on the Sn sites. Thus, the lateral interaction between CO- and OH-adsorbed species facilitates the enhanced effect of CO oxidation on the Pt75Sn25 catalyst, leading to a high-level of CO tolerance in methanol oxidation. Without any further treatment, the pulse MP synthesis emerges as a potential approach in preparing Pt–Sn catalysts with excellent catalytic activity and long-term durability for fuel cell applications.

Acknowledgment The authors are very grateful for the financial support from the National Science Council of Taiwan under the contract NSC 101-2628-E-155-001-MY3.

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Table 1. Characterization data of Pt–Sn/CNT electrocatalysts.

Catalyst

Catalyst loading a

Pt:Sn ratio b

dXRD c

ECSA d

No.

(mg cm-2)

(at%)

(nm)

(m2 g-1).

Pt100

0.26

100:0

2.72

40.1

Pt75Sn25

0.21

79:21

3.06

50.7

Pt50Sn50

0.22

53:47

3.33

26.1

Pt25Sn75

0.23

30:70

3.66

14.2

a

Pt–Sn catalyst loading on the surface CNT determined from TGA analysis.

b

Atomic ratio of Pt to Sn determined from EDS analysis.

c

Crystalline size of Pt–Sn catalysts calculated from Scherrer’s formula, using Eq. (1).

d

Electrochemical surface area determined from the CV profiles at the first cycle with a sweep rate of 30 mV s-1, using Eq. (2).

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Table 2. Onset potential, forward potential, and mass current density of different Pt–Sn catalysts in electrooxidation of methanol at the first CV scan.

Catalyst

Onset Potential

Forward Potential

JF

No.

(V vs. Ag/AgCl)

(V vs. Ag/AgCl)

(A g-1).

Pt100

0.19

0.79

2.77

Pt75Sn25

0.18

0.68

3.54

Pt50Sn50

0.20

0.69

1.13

Pt25Sn75

0.21

0.69

0.57

31



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Page 32 of 45

Table 3. Mass current density and Jdirect/Jindirect ratios of different Pt–Sn catalysts in electrooxidation of formic acid.

Catalyst

Oxidation Peak Current Jdirect (A g-1)

Jindirect (A g-1)

Jdirect/Jindirect (%).

Pt100

13.3

67.3

19.8

Pt75Sn25

16.7

79.5

21.0

18.1

42.2

No. 1st cycle

Pt50Sn50

7.65

Pt25Sn75

0.167

0.885

18.8

200th cycle Pt100

12.1

31.3

38.7

Pt75Sn25

21.2

37.1

57.1

Pt50Sn50

5.12

8.01

63.9

Pt25Sn75

0.312

0.432

72.2

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Figures Caption Fig. 1.

(a) FE-SEM image, (b) TEM micrograph, (c) selected-area electron diffraction pattern, and (d) particle size distribution of Pt75Sn25 catalyst onto the surface of CNTs.

Fig. 2.

TGA curves of different Pt−Sn catalysts at a heating rate of 10 °C min-1 with the temperature range of 30−1000°C.

Fig. 3.

Typical XRD patterns of different Pt−Sn catalysts on CNT support.

Fig. 4.

Cyclic voltammograms of different Pt−Sn catalyst electrodes: (a) Pt100 and (b) Pt75Sn25 samples, in 1 M H2SO4 under N2 at various sweep rates.

Fig. 5.

The ECSA value of different Pt−Sn catalysts as a function of scanning cycle number under the operating conditions (electrolyte: 1 M H2SO4, atmosphere: N2, scan rate: 30 mV s-1, and potential range: -0.2 V to 0.8 V vs. Ag/AgCl).

Fig. 6.

LSV test of different Pt−Sn catalyst electrodes in 1 M H2SO4 at a sweep rate of 30 mV s-1 under high-purity O2 atmosphere.

Fig. 7.

Cyclic voltammograms of different Pt−Sn catalyst electrodes: (a) Pt100 and (b) Pt75Sn25 samples, in 0.5 M H2SO4 + 0.5 M CH3OH at a sweep rate 30 mV s-1 with 1−200 cycles.

Fig. 8.

Amperometric curves of different Pt−Sn catalyst electrodes in 0.5 M

33



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H2SO4 + 0.5 M CH3OH at 0.45 V vs. Ag/AgCl. Fig. 9.

Cyclic voltammograms of different Pt−Sn catalyst electrodes: (a) Pt100 and (b) Pt75Sn25 samples, in 0.5 M H2SO4 + 0.5 M HCOOH at a sweep rate 30 mV s-1 with 1−200 cycles.

Fig. 10.

Amperometric curves of different Pt−Sn catalyst electrodes in 0.5 M H2SO4 + 0.5 M HCOOH at 0.7 V vs. Ag/AgCl.

34


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Page 35 of 45

(a)

(b)

100 nm

5 nm 50

(c)

(d) 40

Frequency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

20

10

0 2

Fig. 1.

2.4

2.8 3.2 3.6 Particle size (nm)

Hsieh et al., 2013.

35


4

4.4


120

Pt100 Pt75Sn25 Pt50Sn50 Pt25Sn75

100

Weight loss (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 45

80

60

40

20

0 0

200

Fig. 2.

400 600 Temperature (oC) Hsieh et al., 2013.

36


800

1000

(311)

(220)

(200)

(111)

Pt100 Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(002)

Page 37 of 45

Pt75Sn25

Pt50Sn50

Pt25Sn75 20

30

40

Fig. 3.

50 60 2θ / degrees

70

Hsieh et al., 2013.

37


80

90


40 30 mV s-1

(a)

10 mV s-1 5 mV s-1

Current density (A g-1)

20

1 mV s-1

0

-20

-40 -0.2

0

0.2 0.4 0.6 Potential (V vs. Ag/AgCl)

0.8

40

(b) Current density (A g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 45

30 mV s-1 10 mV s-1 5 mV s-1

20

1 mV s-1

0

-20

-40 -0.2

0

Fig. 4.


Hsieh et al., 2013.

38


0.8

Page 39 of 45

60

Pt75Sn25

50

ECSA (m2 g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pt100

40

30

Pt50Sn50

20 Pt25Sn75 10 200

400 600 Cycle number Fig. 5.

Hsieh et al., 2013.

39


800

1000


0 Pt25Sn75 -1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 45

Pt50Sn50 -2 Pt100 -3 Pt75Sn25 -4

-5 0.5

0.6 0.7 Potential (V vs. Ag/AgCl) Fig. 6.

Hsieh et al., 2013.

40


0.8

Page 41 of 45

4

(a)

1st


3

2

200th 1

0

Backward Forward peak peak

-1 0

0.2


1

4

(b)

1st 3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

200th 1

0 Backward peak

Forward peak

-1 0

0.2

Fig. 7.


Hsieh et al., 2013.

41


1


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 45

Pt100 Pt75Sn25 Pt50Sn50 Pt25Sn75

2

1

0 0

200

Fig. 8.

400 600 Time (s) Hsieh et al., 2013.

42


800

1000

Page 43 of 45

120


(a) 80

1st

40

200th

0

-40 0

0.2


1

120

(b)

1st 80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200th 40

0

-40 0

0.2

Fig. 9.

0.4 0.6 Potential (V vs. Ag/AgCl)

Hsieh et al., 2013.

43


0.8

1


25 Pt100 Pt75Sn25 Pt50Sn50 Pt25Sn75

20 Current density (A g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 45

15

10

5

0 0

200

400 600 Time (s)

Fig. 10. Hsieh et al., 2013.

44


800

1000

Page 45 of 45

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Table of Content Graphics

An appropriate amount of Sn dopants (25 at%) facilitates not only the catalytic activity but also the long-term anti-poisoning ability, as compared with pure Pt catalyst. The improved performance of Pt−Sn alloy catalyst is attributed to the bi-functional mechanism of bimetallic catalysts. Thus, the introduction of Sn offers one pathway to strip CO from the Pt–CO sites, thereby raising the CO tolerance.


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