Microwave-assisted polyol synthesis of Pt–Zn electrocatalysts on carbon nanotube electrodes for methanol oxidation

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 2 7 6 5 e2 7 7 2

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Microwave-assisted polyol synthesis of PteZn electrocatalysts on carbon nanotube electrodes for methanol oxidation Chien-Te Hsieh *, Wei-Min Hung, Wei-Yu Chen, Jia-Yi Lin Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze University, Taoyuan 32003, Taiwan

article info

abstract

Article history:

Bimetallic PteZn catalysts with high and stable electrochemical activity towards sulfuric

Received 24 August 2010

acid and methanol oxidation were synthesized by microwave-assisted polyol (MP) method.

Received in revised form

A catalytic chemical vapor deposition was used to directly grow multi-layered carbon

2 November 2010

nanotubes (CNTs) on carbon paper substrate. The as-grown CNT forest serves as a support

Accepted 9 November 2010

for the PteZn catalysts having a mean size of 3e5 nm. The catalytic activities of the sup-

Available online 30 December 2010

ported PteZn catalysts toward acid electrolyte and methanol oxidation were examined by cyclic voltammetry test with potential cycling. Experimental results confirmed that two-

Keywords:

stage MP synthesis enables the improvement of electrochemical activity, antipoisoning

PteZn catalysts

ability and long-term durability of the binary catalyst. This improvement can be attributed

Carbon nanotubes

to the bifunctional mechanism of the binary catalysts: the Zn content serves as

Electrochemical activity

a promoting center for the generation of ZneOH species, and more Pt sites are thus

Fuel cells

available for methanol oxidation. Accordingly, the PteZn/CNT catalyst, prepared by the MP

Microwave-assisted reduction

approach, displays a potential candidate for fuel cell application due to its easy fabrication (6 min), low cost and no additional reduction process. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Over the past twenty years, proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) have been intensively researched. They are considered as possible power sources for portable electrical devices and electric vehicles due to their effective energy conversion and high power density [1]. Platinum is known as an efficient metal electrocatalyst for oxygen reduction reaction and methanol oxidation reaction. However, its wide application is hindered by the high cost. It is generally recognized that the replacement of Pt catalysts from electrodes [2] and the improvement of Pt electrocatalytic activity [3] are the two key factors that influence the commercialization of fuel cells. Recently, it has been observed that the alloys of secondary metals such as Zn [4], Fe [5,6], Co [7e10], Ni [11] and Sn [12] with platinum,

display significantly higher electrocatalytic activities toward the oxygen reduction reaction than pure Pt in low-temperature fuel cells. These results confirm that the presence of a secondary element could contribute to decrease in costs associated with Pt. However, the conversion of chemical energy into electricity in fuel cells still requires the development of better electrocatalysts to improve the cell performance. The other aspect of improving this activity is related to electrocatalysts having nanometer size and high dispersion. Carbon-supported Pt-based catalysts are frequently used for practical fuel cell applications. A recent development in the optimization of Pt-based catalysts and high accessible surface area carbon supports has been the investigation of carbon nanotubes (CNTs) as replacements for conventional carbon powders [4]. This can be attributed to the fact that CNTs possess not only high surface area but also good electrical

* Corresponding author. Tel.: þ886 3 4638800x2577; fax: þ886 3 4559373. E-mail address: [email protected] (C.-T. Hsieh). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.11.030

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conductivity and corrosion resistance, inducing high cycling stability. More recently, the direct growth of multi-layered CNTs on carbon paper (CP) and the subsequent selective decorating of Pt catalysts on the CNTs have led to an improvement in the Pt utilization in fuel cells [13e15]. Accordingly, one crucial idea is to directly deposit Pt-based nanoalloy catalysts on CNT/CP composite to prepare gas diffusion electrodes in order to enhance the electrochemical activity for better fuel cell performance. Microwave-assisted polyol (MP) method is one of the efficient approaches to deposit Pt nanoparticles on CNTs [1,16] and carbon powders [17], owing to its energy efficiency, speed, uniformity and simplicity in execution. Nevertheless, studies that focus on the synthesis of binary electrocatalysts are rarely reported. Moreover, a previous study has pointed out that intermetallic PteZn catalyst, synthesized by the reaction of carbon-supported Pt with Zn vapor, exhibits high activity towards formic acid and methanol oxidation [4]. However, the applicability of PteZn electrocatalysts in fuel cells still requires a better understanding. Within this scope, the present work intends to put forward a process for the fast deposit of PteZn electrocatalysts on CNT/CP composite, using one- and two-stage MP methods. Furthermore investigation about the composition, activity and durability with different preparation stages will carry out in this study. The total deposition period during the MP method takes only ca. 6 min and the resulting intermetallic catalysts show a homogeneous dispersion. A comparison of the deposition stage in the MP

method (i.e., one- or two-stage) was made to characterize the electrochemical activity and cycleability of as-deposited PteZn catalysts, based on cyclic voltammetry measurements in sulfuric acid and methanol electrolytes.

2.

Experimental method

2.1.

MP synthesis of electrocatalysts

The procedure for the direct growth of CNTs on CP substrate has been reported elsewhere [18,19]. The CNTs were directly grown on CP (TGP-H-090, Toray) through a catalytic chemical vapor deposition (CVD) process using Ni nanoparticles as the catalyst for the CNT growth. This synthesis technique enabled a large number of CNTs grown on the CP surface, giving a CNT/CP composite. Bimetallic PteZn catalysts were synthesized by microwave heating of ethylene glycol solutions of metallic precursor salts. To functionalize CNTs, the as-prepared CNT/CP composites were chemically oxidized by 1 M nitric acid at 85  C for 1 h. One-stage MP procedure for depositing binary PteZn catalysts on the CNTs was described as follows. The oxidized CNT composites with an area of 5  5 cm2 were impregnated with a Pt-containing solution in a beaker. The ionic solution consisted of 1 ml of 0.04 M PtCl4, 1 ml of 0.12 M Zn(NO3)2, 1 ml of 0.04 M KOH, and 30 ml of ethylene glycol. The beaker was then placed in the center of a household microwave oven and then heated for 6 min

Fig. 1 e FE-SEM micrographs for (a) pure CP, (b) CNT/CP, and (c) PteZn catalysts-supported CNTs. (d) HR-TEM image for PteZn catalysts-supported CNTs.

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120

Pt-CNT PtZn-CNT-m PtZn-CNT

100

80 Weight (%)

under microwave power of 720 W. The metal-coated CNT/CP samples were then separated from the solution and dried at 105  C in a vacuum oven overnight. Now we turned to the two-stage MP synthesis for synthesizing PteZn catalysts. The period for each stage was set at 3 min. In each stage, two kinds of ionic solutions were prepared; (i) at 1st stage for Pt deposition: 1 ml of 0.04 M PtCl4, 1 ml of 0.04 M KOH, and 30 ml of ethylene glycol and (ii) at 2nd stage for Zn deposition: 1 ml of 0.12 M Zn(NO3)2, 1 ml of 0.04 M KOH, and 30 ml of ethylene glycol. The microwave heating for each stage was carried out at 720 W for 3 min. After the two-stage deposition of PteZn catalysts, the bimetallic catalysts were also dried at 105  C in a vacuum oven overnight. To inspect the effect of deposition stage on the electrochemical activity, three types of catalysts were prepared by the MP methods with different metallic solutions: (i) pure Pt ions (one-stage), (ii) mixed Pt and Zn ions (one-stage), and (iii) Pt ions (1st stage) and then Zn ions (2nd stage), designated to Pt-CNT, PteZn-CNT-m, and PteZnCNT, respectively.

60

40

20

0 0

200

400

600

800

1000

1200

Temperature ( C) Fig. 2 e TGA curves for different catalysts.

2.2.

Characterization of electrocatalysts

The microstructures of the electrocatalysts, obtained from the MP synthesis, were characterized by using field-emission scanning electron microscope (FE-SEM, JEOL JSM-5600) and high-resolution transmission electron microscope (HR-TEM, JEOL, JEM-2100). The HR-TEM observation was carried out using a microscope operating at 200 kV. The alloy-attached CNT samples for the analysis were prepared by ultrasonically dispersing the CNTs in ethanol. A drop of the suspension was applied onto a copper grid and was dried in air. A thermogravimetric analyzer (TGA, Perkin Elmer TA7) was used to analyze the amount of catalysts deposited on CNT/CP composites. The TGA analysis was conducted under an air atmosphere with a heating rate of 10  C/min, ramping between 30  C and 1000  C.

2.3.

600 cycles, examining the activity and durability of the binary PteZn catalysts in acid electrolyte. Electrooxidation of methanol on the CNT electrodes was performed at an ambient temperature using 0.5 M H2SO4 þ 1 M CH3OH as the electrolyte solution. A Pt wire was used as the counter electrode, and an Ag/AgCl electrode was used as the reference. The methanol oxidation of the catalyst electrodes was carried out in the potential range of 0.2 to 1.0 V vs.

a

b

(111)

C Pt Zn

Electrochemical measurements of electrodes 38

Three types of CNT/CP composites decorated with catalysts were used to fabricate electrodes for CV measurement. The CV measurement of CNT-based electrodes was carried out within in the potential range from 0.2 V to 0.8 V vs. Ag/AgCl at an ambient temperature using 1 M H2SO4 as the electrolyte solution. In this study, Pt wire and Ag/AgCl electrode served as counter and reference electrodes, respectively. The working electrodes were constructed by pressing the CNT/CP composites onto stainless steel foil, served as current collector. The potential scan rate and scan number were set at 50 mV/s and

39

40

41

42

Pt

2θ (degrees)

PtZn-m

PtZn

Table 1 e Catalyst loading, atomic composition, and ESA of different electrocatalysts determined from TGA, EDX, and CV measurements, respectively. The total period during the MP process was 6 min. Catalyst Pt-CNT PteZn-CNT-m PteZn-CNT

20

30

40

50

60

70

80

90

Loading (wt.%)

Pt:Zn ratio

ESA (cm2/mg)

2 θ (degrees)

5.4 5.5 7.3

100:0 45:55 52:48

109.2 60.8 120.1

Fig. 3 e (a) X-ray diffraction patterns of different types of Pt, PteZn and PteZn samples supported at (CNTs, (b) Pt 111) for different types of Pt, PteZn and PteZn samples.

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Ag/AgCl with a sweep rate of 50 mV/s. All electrochemical measurements in the present work were conducted under N2 atmosphere at ambient temperature.

3.

Results and discussion

3.1.

Characterization of electrocatalysts

Fig. 1(a) and (b) shows FE-SEM images of untreated and CNT-decorated carbon fiber, respectively. The CP substrate is made of fibers having a diameter of approximately 8e10 mm and the surface of the carbon fiber is clean before the CNT deposition. After the CVD treatment, a large amount of coiled CNTs are attached to the surface of carbon fiber, forming CNT/ CP composites. The as-grown CNTs have an average diameter of 20e50 nm and length of several micrometers. Fig. 1(c) displays as-grown PteZn nanoparticles via MP deposition over oxidized CNTs, i.e., PteZn-CNT sample. The rough surface

a

with nanosized spots arises from the small domain size of PteZn nanoalloys. The alloy catalysts also show a homogeneous dispersion onto the nanotubes. This observation indicates that the MP approach enables the easy dispersion of the nanosized binary catalysts. The HR-TEM photograph of PteZn-CNT sample, as shown in Fig. 1(d), reflects that the intermetallic PteZn particles are typically of domain sizes 3e5 nm. The nanoalloys, apparently covering the sidewall of CNTs, appear as lattice stacking layers showing highly crystalline structure. Energy-dispersive X-ray (EDX) was used to analyze the atomic composition of the intermetallic alloys, and the results are listed in Table 1. This clearly indicates that Pt-CNT, as expected, has pure Pt content, whereas there are binary compositions in the other bimetallic PteZn catalysts. It is interesting to note that the deposition stage of the MP process seems to have a minor influence on the atomic composition. The ratios of Pt to Zn elements for both PteZn catalysts approximate to 1:1. However, it can be inferred that the

b

12

4 Cur rent de ns it y (A/g Pt )

8 Current dens it y (A/g Pt )

6

4

0

-4 1st cycle 200th cycle 400th cycle 600th cycle

-8

-0.2

0 0.2 0.4 0.6 Potential (V vs. Ag/AgCl)

c

0.8

0 -2 -4 1st cycle 200th cycle 400th cycle 600th cycle

-6 -8

-12 -0.4

2

1

-0.4

-0.2

0 0.2 0.4 0.6 Potential (V vs. Ag/AgCl)

0.8

1

15

C urre nt density (A /g Pt)

10 5 0 -5 -10 1st cycle 200th cycle 400th cycle 600th cycle

-15 -20 -0.4

-0.2

0 0.2 0.4 0.6 Potential (V vs. Ag/AgCl)

0.8

1

Fig. 4 e Cyclic voltammograms of different electrodes: (a) Pt-CNT, (b) PteZn-CNT-m, and (c) PteZn-CNT. The CVs were performed in 1 M H2SO4 at a sweep rate of 50 mV/s and repeated 600 cycles.

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deposition cycle may alter either the core-shell structure or the alloying degree of binary crystals due to interatomic bond lengths, i.e., PtePt, ZneZn, and PteZn pairs. The homo- or hetero-junctions would result in a disorderly arrangement of binary nanocrystals which can be evidenced through XRD results. TGA analysis was employed to determine the catalyst loading on CNT/CP composites, as shown in Fig. 2. The weight loss in the TGA curve indicates the thermal stability of CNTs and CP, and the residual weight may be attributed to the metallic content. A major weight loss occurs in the temperature region of 500e800  C, assigning to the carbon gasification (i.e., amorphous carbon and multi-layered CNTs) [20]. The residual weights, corresponding to the metallic loading for all catalyst samples, are also shown in Table 1. The Pt-based catalyst loading falls in the region of 5.4e7.3 wt.%. It is worth noting that PteZn-CNT sample has a catalyst loading higher than the other two samples. This can be ascribed to the fact that the two-stage MP method enables a greater amount of metallic deposition due to the step-by-step formation of binary alloys on the sidewalls of CNTs. In contrast, a competitive reduction possibly takes place during the one-stage microwave heating, resulting in a lower amount of binary catalysts attached to the CNTs. To indentify the crystalline structure, Fig. 3 (a) shows the result of XRD for each pair of the bimetallic nanoparticles deposited on CNTs. It is general recognized that pure Pt has a face-centered cubit (fcc) structure. There are peaks at 2q z 40 and 47 which can be characterized with the (111) and (200) planes, respectively [21], of the fcc structure of platinum. Additionally, some other peaks appeared at 2q z 42.5 and 64 reflect the presence of Zinc deposited on CNTs [22]. After the addition of Zinc metal, the diffraction peaks of binary PteZnm or PteZn catalyst lead to broaden and shift to high 2q values [23]. As shown in the insert picture of Fig. 3(b), the 2q of the (111) peak for PteZn-m or PteZn catalyst higher than 40 shows a slight shift, when compare with the pure Pt at 39.9 .

3.2.

capacitive charge in the double layers of Pt-based catalysts, CNTs and CP substrate. The total charge transfer (QT) can be determined by integrating the CVs in the relevant potential region 1 QT ¼ n

Z0:2 ðID  IA Þ$dE

(2)

0:2

where n is the sweep rate, ID and IA are the specific current of desorption and adsorption respectively, and E is the potential. The SESA values of PteZn catalysts can thus be obtained from SESA ¼

QH QPt

(3)

where QPt is considered to be 0.21 mC/cm2, determined from the electrical charge associated with monolayer adsorption of hydrogen on Pt. For the first cycle, the SESA values of different catalysts were calculated as shown in Table 1. The values of SESA observe the following order: PteZn-CNT (120.1 cm2/mg) > Pt-CNT (109.2 cm2/mg) > PteZn-CNT-m (60.8 cm2/mg). Generally, the SESA values can be considered as an important estimation of surface activity. The electrocatalytic activity of PteZn-CNT catalyst, prepared by the two-stage MP process, is apparently improved compared to the other two catalysts. On contrary, PteZn-CNT-m electrode prepared by one-stage MP process has the lowest activity toward acid electrolyte. The above result discloses two crucial factors: (i) deposition stage and (ii) binary catalyst, which significantly enhance the activity in acid electrolyte and will be discussed later. Fig. 5 shows the variation of SESA values with scanning cycle number, confirming the cycling stability of the electrocatalysts. This figure reveals that there are two obvious declines, after cycling, for both SESA values of the catalysts (i.e., Pt-CNT and PteZn-CNT-m samples), whereas the PteZn-CNT catalyst maintains a stable activity within 600 cycles. This result demonstrates that PteZn-CNT catalyst possesses not only an enhanced activity, but also an improved cycling

Electrochemical activity of electrocatalysts in H2SO4 200

1 QH ¼ ðQT  QDL Þ 2

Pt-CNT PtZn-CNT-m PtZn-CNT

160

SESA (cm2 mg-1 Pt)

The cycle voltammograms in 1 M H2SO4 for the PteZn electrocatalysts at a sweep rate of 50 mV/s are collected, as shown in Fig. 4(a)e(c). The CV scans from an open circuit potential and sweeps within the potential region between 0.2 V and 0.8 V vs. Ag/AgCl. Basically, all catalysts exhibit typical hydrogen adsorptionedesorption peaks within 0.2e0.2 V vs. Ag/AgCl. The CV scans were repeated at 600 cycles in order to inspect electrochemical activity and cycling stability. The CV features clearly show the different electrochemical activities and stabilities of the electrocatalysts. The integrated area within the potential range of 0.2e0.2 V can be used to evaluate the electrochemical surface area (SESA) on the surface of Pt-based catalysts. To obtain the SESA value, first the specific charge transfer due to hydrogen adsorption and desorption (QH) should be obtained as [24],

120

80

40

0 0

(1)

where QT is the total specific charge transfer in the hydrogen adsorptionedesorption potential region, and QDL is the specific

100

200 300 400 Cycle number

500

600

Fig. 5 e The SESA value as a function of cycle number for different catalysts.

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capability toward acid electrolyte. The steady activity of PteZn alloy catalyst can be achieved by Pt-rich skin on the catalysts, as has been reported for surfaces of PteCo alloy [3,5]. It is of interest that PteZn-CNT-m catalyst has a binary composition, but a poor activity and cycling stability. This leads us to the conclusion that one-stage deposition of binary elements under microwave irradiation generates binary catalysts with low alloying degree due to competitive reduction during the short period. Two possible explanations for the activity decay can be proposed: some Zn islands are easily leached from (i) the Pt surface and (ii) the sidewall of CNTs in H2SO4 solution. The partial leaching of Zn is capable of making the catalyst unstable and accelerating the separation of PteZn particles from the carbon support, thereby reducing the stability.

3.3.

Methanol oxidation of electrocatalysts

The other electrochemical performance of as-prepared cycle PteZn catalyst electrodes was examined by cyclic voltammetry in 0.5 M H2SO4 þ 1 M CH3OH solution, as shown in Fig. 6

a 200

(a)e(c). The CV measurement was swept at a scanning rate of 50 mV/s for 100 cycles. A typical feature of methanol oxidation generally appears in the CV curves. The forward scan is attributed to methanol oxidation, forming Pt-adsorbed carbonaceous intermediates, e.g., CO and CO2. However, the presence of PteCOads intermediates results in the activity decay of electrocatalysts [25]. The backward oxidation peak is ascribed to the additional oxidation of the adsorbed carbonaceous species to CO2. Generally, three indices e onset potential, forward potential and the ratio of the forward peak current (IF) to the backward peak current (IB) e can be considered as the estimates of catalyst activity for ethanol oxidation. These indices, for all catalysts, are summarized as shown in Table 2. These catalysts show a similar onset potential of ca. 0.42 V toward oxidation, while they have obvious differences for forward potential and IF/IB ratio. This addition of Zn element tends to lower the forward oxidation potential peak significantly, possibly depending upon the nature of PteZn/CNT catalysts. The IF/IB ratio can be an important index of the conversion of CO2

b 200 1st cycle 1st cycle

160

160

120

C ur re nt (A/g Pt)

Current (A/g Pt )

100th cycle 100th cycle

80

40

0

120

80

40

0

-40

-40 -0.4

-0.2

0 0.2 0.4 0.6 0.8 Potential (V vs. Ag/AgCl)

1

1.2

-0.4

-0.2

0 0.2 0.4 0.6 0.8 Potential (V vs. Ag/AgCl)

1

1.2

c 200 1st cycle

C ur re nt (A/g Pt)

150

100th cycle 100

50

0

-50 -0.4

-0.2

0 0.2 0.4 0.6 0.8 Potential (V vs. Ag/AgCl)

1

1.2

Fig. 6 e Cyclic voltammograms of methanol electrooxidation in 0.5 M H2SO4 D 1 M CH3OH on different electrodes: (a) Pt-CNT, (b) PteZn-CNT-m, and (c) PteZn-CNT, at a sweep rate of 50 mV/s.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 2 7 6 5 e2 7 7 2

Table 2 e Onset potential, peak potential, and IF/IB ratios of different electrocatalysts. Catalyst No.

Onset potential (V vs. Ag/AgCl)

Forward potential (V vs. Ag/AgCl)

IF/IB after 100 cycles

Pt-CNT PtZneCNT-m PtZneCNT

0.42 0.42 0.42

0.80 0.71 0.68

0.94 1.08 1.51

amount to CO amount, oxidized from methanol fuel [26]. Basically, a higher IF/IB ratio can be applied to measure the catalyst tolerance to CO poisoning [24], i.e., the fraction of catalyst surface that is not poisoned by CO adsorption. As shown in Table 2, the IF/IB values at 100th cycle show the following order: PteZn-CNT (1.51) > PteZn-CNT-m (1.08) > Pt-CNT (0.94). This finding indicates that both Pt-CNT and PteZn-CNT-m catalysts have lower CO tolerance and poor durability compared to PteZn-CNT catalyst. Accordingly, this reflects that PteZn-CNT catalyst displays the best ability to resist CO poisoning among these catalysts. On the basis of these results, we can conclude that the PteZn-CNT catalyst exhibits better electrochemical activity, antipoisoning ability and long-term cycleability among these catalysts. This improvement in catalytic activity and durability in the case of methanol oxidation can be ascribed to the distribution of surface atoms in the PteZn catalysts. Based on the bifunctional theory [27], an efficient catalyst favors CO adsorption on Pt, and OH formation takes place on the second metal. Since PteZn-CNT catalyst provides a large number of PteZn pairs (i.e., high alloying degree), the Zn content acts as a promoting center for the generation of ZneOH species, inducing more Pt sites for methanol oxidation. Thus, the Pt sites are prone to methanol dehydrogenation, while the dehydrogenation of water is more facile on the Zn sites [28]. Therefore, the binary combination yields a good overall activity and CO tolerance for methanol oxidation on the PteZn alloy. This demonstrates that the design of binary catalysts opens up a possibility for the replacement of expensive PteRu alloy in DMFCs.

4.

Conclusions

This work employed an efficient microwave-assisted approach to prepare bimetallic PteZn electrocatalysts on the CNT/CP composites. This MP synthesis offers many advantages like convenience, short period (6 min) and no thermal reduction. The binary PteZn alloys having an average size of 3e5 nm were found to have an equal atomic Pt/Zn ratio, determined by the EDX analysis. The CV measurements reflected that the PteZn catalysts are electrochemically active in hydrogen adsorption/desorption and methanol oxidation. The deposition sequence plays an important role in affecting the electrochemical activity and durability of PteZn catalysts. On the basis of experimental results, we see that the PteZnCNT catalyst, prepared by two-stage MP synthesis, displays better electrochemical activity, antipoisoning ability and longterm durability than the other two catalysts. The steady activity of PteZn alloy catalyst can be achieved due to the

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Pt-rich skin on the binary catalysts, proven by the bifunctional mechanism of bimetallic catalysts. Incorporating the design of alloy catalysts with effective MP deposition offers a commercial feasibility to replace traditional PteRu/CNT catalyst in case of DMFCs.

Acknowledgements The authors are very grateful for the financial support from the National Science Council (Taiwan) under the contracts NSC 99-2120-M-155-001 and NSC 99-2221-E-155-078.

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