메탄올 전기산화반응 증진을 위한 PtRuW/C 촉매에서 텅스텐의 효과에 관한 연구

메탄올 전기산화반응 증진을 위한 PtRuW/C 촉매에서 텅스텐의 효과에 관한 연구

노창수^‡⋅손정민*^,†⋅박영권**

전북대학교 수소연료전지공학과, *전북대학교 자원⋅에너지공학과, **서울시립대 환경공학부 (2012년 8월 22일 접수, 2012년 9월 22일 심사, 2012년 10월 19일 채택)

Effect of Tungsten on PtRuW/C Catalysts for Promoting Methanol Electro-oxidation

Chang Soo Noh

, Jung Min Sohn

, and Young-Kwon Park

Department of Hydrogen and Fuel Cells Engineering, Specialized Graduate School, Chonbuk National University, Jeonju 561-756, Korea

*

Department of Mineral Resources & Energy Engineering, Chonbuk National University, Jeonju 561-756, Korea

**

Faculty of Environmental Engineering, University of Seoul, Seoul 130-743, Korea (Received August 22, 2012; Revised September 22, 2012; Accepted October 19, 2012)

PtRuW/C 촉매를 Pt : Ru : W 비를 5 : 4 : 1, 2 : 1 : 1, 1 : 1 : 1, 1 : 2 : 2로 각각 합성하였다. 촉매의 조성은 EDX분석을 통해 이론값과 비슷하다는 것을 확인하였다. TEM분석과 XRD분석으로부터 3.5∼5.5 nm의 균일한 입자 크기 및 결정 질 분포를 가지고 있음을 확인하였다. 유효표면적, 전류밀도, 고유 활성 및 피독률과 같은 전기화학적 특성은 CO 스트 리핑, 선형쓸음 전기량 측정법, 대시간 전류법 등과 같은 방법으로 분석하였다. 이러한 분석으로부터, PtRu

W

/C 를

제외한 촉매는 상업촉매보다 우수한 반응성과 안정성을 가지고 있음을 확인하였다. 이 중 가장 우수한 촉매는 Pt

Ru

W/C

였으며, 121.05 mA⋅m

의 specific activity와 0.01%⋅s

의 피독률을 보였다.

PtRuW/C catalysts were prepared with the different molar ratios of Pt : Ru : W and their compositions were analyzed by energy dispersive X-ray (EDX). The uniform distribution of particles was observed using transmission electron microscopy (TEM). An average crystalline size of 3.5∼5.5 nm was calculated based on x-ray diffraction (XRD) data. The electrochemical properties such as electrochemically active surface areas, current densities, specific activities and poisoning rates, were ana- lyzed via CO stripping, linear sweep voltammetry and chronoamperometry. From the analysis, we observed that ternary alloy catalysts, except PtRu

W

/C, have higher current densities, specific activities and stabilities than those of commercial binary catalysts. Among all in-house catalysts, Pt

Ru

W/C showed the highest specific activity of 121.05 mA ⋅m

and the lowest poisoning rate of 0.01 %⋅s

.

Keywords: methanol electro-oxidation, ternary catalyst, PtRuW, tungsten effect, direct methanol fuel cell

1. Introduction

1)

Fuel cells have attracted much attention for various applications as a next generation power source[1,2]. There have been studies on vari- ous fuel cells as energy sources for mobile devices, such as laptops and mobile phones, including the direct methanol fuel cell (DMFC). It has a high energy density, easily handling as a liquid fuel, as well as low operating temperatures and volumetric energy density; about 10 times larger than those of lithium ion batteries[3,4]. However, this merit

† Corresponding Author: Chonbuk National University Department of Mineral Resources and Energy Engineering 664-14 Duckjin-dong, Duckjin-gu, Jeonju, Jeonbuk 561-756, Korea Tel: +82-63-270-2361 e-mail: [email protected]

‡ 현재 소속: GS 파워(주) Current Address: GS Power Co.)

pISSN: 1225-0112 @ 2012 The Korean Society of Industrial and Engineering Chemistry.

All rights reserved.

of DMFC also results in drawbacks of methanol cross-over and low catalytic activity of anode catalysts for methanol electro-oxidation.

Platinum was initially used for methanol electro-oxidation, but is easily poisoned by intermediate CO. Therefore, the use of binary catalysts, with the introduction of a new metal into the Platinum-base, has been studied[5-9]. Although PtRu is known to be the best catalyst for meth- anol electro-oxidation, improvement is required for its commercial ap- plication to the DMFC.

It is well known that the addition of Ruthenium to Platinum-based catalysts lowers the overpotential of the methanol electro-oxidation reaction via a bifunctional mechanism[10] ;

Pt + CH

OH → Pt-CO

+ 4H

+ 4e

(1)

Ru + H

O → Ru-OH

+ H

+ e

(2)

Pt(CO)

+ Ru(OH)

→ CO

+ Pt + Ru + H

+ e

(3)

The activity of the PtRu catalyst is increased by the bifunctional

공업화학, 제 23 권 제 6 호, 2012

Figure 1. XRD results for in-house and commercial catalysts.

effect, which involves water discharge onto the Ru to produce Ru-OH[10]. From this reaction (2), the main role of Ru is the decom- position of the water. Reaction (3), which produces carbon dioxide, is the only rate determining step for the PtRu catalyst. To increase the catalytic activity of the anode, the introduction of a PtRu binary cata- lyst was found to act as the best catalyst, due to its tolerance to CO, and is widely used in DMFC for methanol electro-oxidation[11].

The platinum and ruthenium used with the precursors for electrode catalyst are very expensive. Also, noble metals as a resource are lim- ited; therefore, new alternative materials for electrode catalysts are required. Thus, studies on more complicated systems, such as ternary and quaternary metals, have also been conducted[12]. These ternary and quaternary catalysts, such as PtRuW[13], PtRuRh[14], PtRuNi[15,17], PtRuIr[18], PtRuFe[19], PtRuIrSn[4], PtRuMoW[20], PtRuRhNi[21]

and PtRuOsIr[22], were shown to have higher methanol oxidation than the PtRu catalyst.

In this work, PtRuW/C ternary catalysts, with the addition of different amounts of tungsten, were prepared using a conventional impregnation method employing NaBH

as a reducing agent and compared with commercial PtRu/C (60 wt%, E-tek). The catalysts were characterized by TEM, energy dispersive x-ray (EDX) and XRD, respectively.

Further, their electrochemical properties were analyzed via CO strip- ping, linear sweep viltammetry and chronoamperometry, respectively, using a three-electrode half cell.

2. Experimental

A conventional impregnation method (reducing agent: sodium bor- ohydride, NaBH

) was used to synthesize the catalysts. Initially, Vulcan XC72R carbon black was dispersed in a mixture of de-ionized water and isopropyl alcohol and then sonicated. Metal precursors were dis- solved in de-ionized water and stirred. H

PtCl

⋅xH

O (Kojima Chem.

Co.), RuCl

(Aldrich Chem. Co.) and WCl

(Aldrich Chem. Co.) were used as the Pt, Ru and W precursors, respectively. The amounts of metal precursors were adjusted to give Pt : Ru : W of different molar ratios; namely 5 : 4 : 1, 2 : 1 : 1, 1 : 1 : 1, and 1 : 2 : 2, with a total metal content in the catalyst of 60 wt%. The mixture was heated to, and maintained at, 80 ℃ for 1 h with stirring. A 0.2 M NaBH

solution was added to reduce the mixture. The resulting solution was mixed thoroughly for 3 h, followed by filtering and washing with hot de-ionized water. The catalysts were then dried overnight in an oven at 80 ℃.

XRD patterns were recorded on a Rigaku DMAX-2500 using a Cu K α radiation source, and TEM images of the in-house catalysts using a JEM2200FS. Moreover, the compositions of the in-house catalysts were determined by EDX analysis using JSM-6400.

Electrochemical measurements were carried out at 25 ℃ using a potentiostat (Bio-Logic, SP-150). A three-electrode half cell, with a Pt-wire, Ag/AgCl (BAS Co., Ltd., MF-2052 RE-5B) and glassy carbon electrode (3 mm diameter, BAS Co., Ltd., MF-2012) used as the coun- ter, reference and working electrodes, respectively.

The glassy carbon working electrode was prepared by the thin-film

method suggested by Schmidt et al.[23] For the fabrication of the working electrode, the catalyst was dispersed in de-ionized water with sonication, and the catalyst suspension then dropped onto the glassy carbon substrate. After drying at room temperature, a 5 wt% Nafion ionomer solution was dropped onto the surface to stabilize the catalyst layer. Further, working electrode of commercial PtRu/C (60 wt%, E-tek) was also prepared by same method compared to properties of our catalysts. The 1 M HClO

and 1 M CH

OH in 1 M H

SO

electrolyte solutions were purged with nitrogen gas (N

) before the experiments.

3. Results and Discussion

Figure 1 shows the X-ray diffraction results of the catalysts prepared using the addition of different mounts of tungsten. In all diffraction results, a broad peak at about 25° marked with C was associated with the (002) plane of the hexagonal structure of carbon black support material.

As can be seen from Figure 1 there were no other distinct reflection peaks, which indicated the presence of Ru and W metals, except hexagonal structure of carbon black, in-house ternary alloy catalysts have approximately 39.5°, 46.1°, and 67.7° of face centered cubic (FCC) crystalline Pt phase, corresponding to Pt (111), (200), and (220) planes, respectively.

Significant changes were observed in spectrogram of PtRu

W

/C catalyst. Firstly, spectrogram of PtRu

W

/C was very smooth than that of other catalysts and was not observed significant Pt and Pt alloy peaks. The behavior of binary and/or ternary alloys with respect to electrocatalysis can be understood in terms of “dilution effects”. Many investigators[24-26] reported that when second and/or third metal of high concentration was added into Pt and Pt alloy catalysts, it could block on active sites of Pt. Judging from this, very smooth spectrogram of PtRu

W

/C is probably due to the amounts of ruthenium and tungsten were adjusted two times higher than of platinum.

Secondly, one additional peak was observed at approximately 2 θ =

34°, and was identified as (200) phase of the tungsten oxide (WO

)

[27]. Furthermore, the diffraction peaks of the in-house catalysts were

slightly shifted to higher 2 θ values than that of commercial PtRu/C,

Table 1. Atomic Ratios of In-house Catalysts and Crystalline Size

Catalysts Determined by EDX (%) Crystalline size (nm)

Pt Ru W XRD

Pt

Ru

W/C 49.0 39.2 11.8 3.5

Pt

RuW/C 51.4 23.6 25.1 4.6

PtRuW/C 35.8 31.3 32.9 4.4

PtRu

W

/C 17.5 43.7 38.8 5.5

(a) (b)

(c) (d)

Figure 2. TEM image for: (a) Pt

Ru

W/C, (b) Pt

RuW/C, (c) PtRuW/C, and (d) PtRu

W

/C.

(a) (b) (c)

(d) (e)

Figure 3. CO stripping result for: (a) Pt

Ru

W/C, (b) Pt

RuW/C, (c) PtRuW/C, (d) PtRu

W

/C, and (e) commercial PtRu/C.

due to the formation of an alloy involving the incorporation of Ru and W into the fcc structure of the Pt[28].

The crystalline sizes of in-house catalysts were calculated using the Debye-Scherrer equation[29] and listed in Table 1.

θ β

λ

2

cos

/ 1

d = k

Equation (1)

Where d is the particle size (nm), λ is the wavelength of X-ray, θ is the angle of maximum peak, β

is the width of the diffraction peak at half height, k is a coefficient of 0.89 to 1.39 (0.9 here).

Obviously, all catalysts had comparatively small crystalline sizes which were approximately 3.5∼5.5 nm and a good alloy formation as observed from the TEM analysis shown in Figure 2. Uniform dispersion of the in-house catalysts, except PtRu

W

/C, was observed for the all catalysts without agglomeration. Moreover, the chemical compositions of the catalysts were determined by EDX analysis and were quite similar to the nominal value.

The CO stripping was measured in a 1 M HClO

solution, with a scan rate of 15 mV ⋅s

, as follows; CO was bubbled through the working electrode for 1 h, while maintaining a constant voltage of 0.1 V

(vs. RHE). The dissolved CO in the electrolyte was purged by nitrogen gas bubbling for 50 min. The CO stripping results for the in-house and commercial (60 wt%, E-tek) catalysts are showed in Figure 3. Significant changes were observed at two points:

Firstly, the potential of the in-house catalysts became more negative

공업화학, 제 23 권 제 6 호, 2012

Figure 4. On-set voltage for the CO oxidation of Pt

Ru

W/C (dash), Pt

RuW/C (dash dot), PtRuW/C (dash dot dot), PtRu

W

/C (dot), and commercial PtRu/C (solid).

Figure 5. Liner sweep voltammetry (LSV) curves for in-house and commercial catalysts for the methanol oxidation in a solution of 1 M H

SO

+ 1 M CH

OH at room temperature.

Table 2. Electrochemical Properties of In-house and Commercial Catalysts Catalysts On-set voltage for

CO oxidation [V]

SEAS [m

(g catal)

]

Current density at 0.5 V [mA cm

]

Mass activity [A(g ⋅Pt)

]

Specific activity [mA m

]

Pt

Ru

W/C 0.243 39.65 5.44 14.24 121.05

Pt

RuW/C 0.187 36.80 3.40 8.90 81.52

PtRuW/C 0.183 18.09 1.42 5.28 69.09

PtRu

W

/C 0.161 13.86 0.34 2.40 21.64

PtRu/C (E-tek) 0.490 42.56 2.79 7.30 57.80

as the amount of tungsten increased. The CO oxidation potential is one of the key properties in determining the catalytic activity for methanol oxidation, as the CO oxidation reaction is a slow step during the methanol oxidation reaction[30]. The on-set voltages of the in-house catalysts for CO oxidation are compared in Figure 4. The on-set voltages of the CO oxidation of Pt

Ru

W/C, Pt

RuW/C, PtRuW/C and PtRu

W

/C, which were all lower than the 0.49 V of commercial PtRu/C, were 0.243, 0.187, 0.183, and 0.161 V, respectively, indicating incorporation of W could lower the CO oxidation potential, as expected; secondly, the appearance of a new peak at a more negative potential on the addition of W. Koper et al. investigated[31] island-covered heterogeneous surfaces Pt-Ru bimetallic alloy predicted two CO stripping peak maxima.

A peak observed at a low potential was assigned to CO oxidation on the Pt sites next to the ruthenium island edge, while a high potential peak was attributed to CO oxidation on the Pt atoms much further away from the Ru sites. Our CO stripping results support these theoretical predictions, although the theoretical model assumes virtually no surface diffusion to generate the well-resolved peak separations.

Furthermore, a CO stripping experiment to evaluate the amounts of CO adsorbed was performed to measure the electrochemically active surface area (EAS). Due to the strong adsorption of CO onto the Pt surface, hydrogen adsorption-desorption on the Pt was completely blocked in the hydrogen region; this indicates the presence of a saturated CO

layer[32]. The electrochemically active surface areas of the catalysts were calculated using Equation (2)[27,33].

) (

420

= ×

Ccm G

S

Q

µ Equation (2)

where Q

is the charge for CO desorption electro-oxidation in micro-coulomb (µC), G represents the summation of catalysts metal loading (µg) in the electrode, and 420 is the charge required to oxidize a monolayer of CO on the catalyst in µCcm

. The calculated S

were 42.56, 39.65, 36.80, 18.09, and 13.86 m

(g⋅catal)

for commercial PtRu/C, Pt

Ru

W/C, Pt

RuW/C, PtRuW/C, and PtRu

W

/C, respectively.

The in-house catalysts mainly showed reduced S

with increasing tungsten content, such as the PtRuW/C and PtRu

W

/C catalysts. The results from this experiment confirmed that exposure of Pt to the surface of the catalyst particles was obstructed by W contents.

The methanol oxidation activity was measured in a solution of 1 M CH

OH + 1 M H

SO

at a scan rate of 15 mV ⋅s

. As shown in Figure 5, the current densities of the in-house catalysts were much larger than that of commercial catalyst, except that of PtRu

W

/C, indicating the Pt

Ru

W/C catalyst had the highest methanol oxidation activity. The electrochemical properties of the catalysts are listed in Table 2.

The current densities were converted into mass and specific activities.

The mass activities at 0.5 V were 14.24, 8.90, 5.28, 2.40, and 7.30 A(g

⋅Pt)

for Pt

Ru

W/C, Pt

RuW/C, PtRuW/C, PtRu

W

/C, and the

commercial PtRu/C catalyst, respectively. Pt

Ru

W/C and Pt

RuW/C

exhibited mass activities of 14.24 A(g⋅Pt)

and 8.90 A(g⋅Pt)

,

which were higher than the 7.30 A(g⋅Pt)

of the commercial PtRu/C

Figure 6. Chronoamperometry curves for in-house and commercial catalysts.

Table 3. Poisoning Rate of In-house and Commercial Catalysts Catalysts Poisoning rate (%s

) ( δ)

Pt

Ru

W/C 0.01

Pt

RuW/C 0.03

PtRuW/C 0.03

PtRu

W

/C 0.04

Commercial PtRu/C (E-tek) 0.02

Figure 7. The effects on the performance in relation to the tungsten addition ratio.

catalyst. The Pt

Ru

W/C catalyst showed a mass activity especially higher; about twice that of the commercial PtRu/C catalyst. For the specific activity, the Pt

Ru

W/C catalyst was still approximately twice that of the PtRu/C catalyst, and both Pt

RuW/C and PtRuW/C also showed higher specific activities than the PtRu/C catalyst.

To investigate the tolerance of the in-house catalysts during the methanol oxidation, chronoamperometry tests were performed for 2000 s in a solution of 1 M CH

OH + 1 M H

SO

. Figure 6 showed the chronoamperometry results for the in-house catalysts at a fixed potential of 0.5 V, which shows the incipient currents for methanol oxidation were higher than those found by linear sweep voltammetry at the same potential, due to the lower amounts of Pt oxides (PtO

) in the catalysts [34]. Furthermore, the potentiostatic current decreased rapidly within a short time; this may have been due to the formation of CO

and other intermediate species, such as CH

OH

, CHO

, and OH

, during the methanol oxidation reaction[35], and the effect of diffusion limitation of reactants on the electrode. The in-house catalysts were gradually oxidized, similarly to that of the commercial catalyst; the poisoning rate ( δ) of catalysts with use over a long time were calculated using Equation (3), [36,37] and are listed in Table 3.

s

dt

dI

I

100

>

⎟ ⎠

⎜ ⎞

⎝

× ⎛

=

δ (% ⋅s

) Equation (3)

Where dt dI

is the slope of the linear portion of the current decay above 500 s and I

is the current at the start of polarization back extrapolated from the linear current decay.

In Table 3, the poisoning rate of the Pt

Ru

W/C catalyst (0.01 % ⋅ s

) was lower than that of the commercial catalyst (0.02 % ⋅s

).

Further, the Pt

RuW/C and PtRuW/C catalysts, except the PtRu

W

/C catalyst showed slightly high poisoning rate than the commercial catalyst, but were confirmed to have comparatively low poisoning rates; 0.03 %⋅s

.

As shown in Figures 3∼7, the anodic efficiency decreased with increasing amount of tungsten. After a 10 mole% of tungsten composition, as in the standard, the poisoning rate (circle) and specific activity (square) was increased and decreased, respectively.

The above results indicated that by the addition of tungsten, the PtRuW ternary alloy structure was observed to have the resistance to CO poisoning and promote electrical performance to methanol electro-oxidation. Further investigation for understanding the detailed mechanism is carried out.

4. Conclusions

PtRuW/C catalysts were prepared with the addition of different amounts of tungsten using the impregnation method employing NaBH

as a reducing agent. The catalytic activities of the in-house catalysts were measured via electrochemical experiments, including CO stripping, linear sweep voltammetry and chronoamperometry, and compared to commercial PtRu/C.

From the CO stripping, the catalysts were confirmed to move to more negative potentials as the amount of tungsten was increased.

Furthermore, the maximum specific activity of the catalysts for methanol oxidation was found with Pt

Ru

W/C; 121.05 mA⋅m

. And poisoning rate of Pt

Ru

W/C catalyst (0.01 % ⋅s

) was lower than that of commercial catalyst (0.02 % ⋅s

). We realized in this experiment that Pt

Ru

W/C catalyst gave a higher performance than commercial PtRu catalyst.

Acknowledgements

This research was supported by Basic Science Research Program

through the national research foundation of Korea (NRF) funded by the

공업화학, 제 23 권 제 6 호, 2012

Ministry of Education, Science and Technology (2009-0068074) and the Program of Regional Innovation Center for the fuel-cell technology funded by the Ministry of Knowledge Economy of the Korean Government.

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