Ir–V nanoparticles supported on microstructure controlled carbon nanofibers as electrocatalyst for oxygen reduction reaction

Electrochemistry Communications 12 (2010) 27–31

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Ir–V nanoparticles supported on microstructure controlled carbon nanofibers as electrocatalyst for oxygen reduction reaction Jun-Sheng Zheng a,b, Xi-Zhao Wang a,b, Jin-Li Qiao a,b, Dai-Jun Yang a,b, Bing Li a,b, Ping Li c, Hong lv a,b, Jian-Xin Ma a,b,* a b c

Clean Energy Automotive Engineering Center, 4800 Caoan Road, Tongji University (Jiading Campus), Shanghai 201804, China School of Automotive Studies, 4800 Caoan Road, Tongji University (Jiading Campus), Shanghai 201804, China State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

a r t i c l e

i n f o

Article history: Received 22 September 2009 Received in revised form 16 October 2009 Accepted 20 October 2009 Available online 23 October 2009 Keywords: Ir–V nanoparticles Carbon nanofibers Electrocatalyst Oxygen reduction reaction Electrochemical properties

a b s t r a c t Ir–V nanoparticles supported on microstructure controlled carbon nanofibers (CNFs) or on carbon black, Vulcan XC-72 (XC-72), have been synthesized via chemical reduction, and the oxygen reduction reaction (ORR) properties of catalysts are investigated in this paper. The physico-chemical properties are characterized by high resolution transmission electron microscope (HRTEM), N2 physisorption and electrochemical analysis. HRTEM results show that the metal nanoparticles are separated on carbon support with well-controlled particle size, dispersity, and composition uniformity. Moreover, the metal nanoparticles on CNFs have a smaller size than those on XC-72. Cyclic voltammetric analysis reveals that Ir–V/ CNFs exhibits a higher ORR activity than Ir–V/XC-72, and this may be associated with the smaller metal nanoparticles and the stronger metal-support interaction of Ir–V/CNFs. Linear sweep voltammetric analysis at different rotation rates proves that ORR on the Ir–V/CNFs electrode is a 4e process. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Oxygen reduction reaction (ORR) is involved in a number of electrochemical processes [1], and has been a research focus in recent decades when alternative resource utilization and energy conversion efficiency are of primary concern [2]. Pt is considered as the most promising ORR catalyst for its high catalytic activity, and the carbon supported Pt nanoparticles are the most widely used catalyst for ORR [3]. Unfortunately, the world reserves of Pt are relative scarcity, consequently, the replacement of Pt with a less expensive material or metal alloy is become very considered, and different materials, such as Pd, Fe-based catalysts [4] are extensively investigated. Ir [5] has been proposed as good promoters in Pt because of their excellent oxygen evolution, high activity on COads in acidic media [6]. Good performances have been reported for ethanol oxidation on Ir–Sn [7] and formic acid oxidation on Pd–Ir [8]. In a recent work, we had reported that Ir–V nanoparticles on Vulcan XC-72 (XC-72) as hydrogen oxidation catalyst, and revealed this catalyst had a significant activity [9]. It is well known that carbon support is of importance for catalyst performance. Carbon black, such as XC-72, has been extensively studied as the most widely used support because of its * Corresponding author. Address: School of Automotive Studies, 4800 Caoan Road, Tongji University (Jiading Campus), Shanghai 201804, China. Tel.: +86 21 6958 9480; fax: +86 21 6958 9121. E-mail address: [email protected] (J.-X. Ma). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.10.028

good compromise between electronic conductivity and surface area [10]. Since its invention for the first time, CNTs have attracted attentions and the catalysts supported on CNTs have higher catalytic activities than those supported on traditional carbon materials [11]. Whereas, CNTs have small surface area and weak interactions with the supported metals, which may restrict the improvement of catalytic activity. These shortcomings can be overcome by using carbon nanofibers (CNFs) as the support because they have large surface area and exposed more edge atoms. Particularly, the platelet CNFs (p-CNFs), which have a higher ratio of edge atoms to basal atoms than other CNFs, provide a means to adjust the deposition of and interaction with the supported metal. Bessel et al. [12] revealed that 5 wt.% Pt supported on p-CNFs was as active as 25 wt.% Pt supported on XC-72. Similarly, we found that Pd nanoparticles on p-CNFs had higher ORR activities than those on fish-bone CNFs or activated carbon [13]. In this work, Ir–V nanoparticles supported on p-CNFs and on XC-72 are synthesized, and the physico-chemistry properties and the ORR properties of Ir–V/CNFs and Ir–V/XC-72 are investigated.

2. Experimental 2.1. Catalyst preparation CNFs were synthesized by decomposition of CO on iron catalyst in a fixed-bed quartz reactor by catalytic chemical vapor

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deposition (CCVD). Details of CNF synthesis procedure were also described in Ref. [13]. Ir–V nanoparticles supported on CNFs or on XC-72 were prepared via an ethylene glycol chemical reduction method with IrCl36H2O as the iridium precursors and NH4VO3 as the vanadium precursors, and details of the catalyst synthesis procedure were described in Ref. [14].

3. Results and discussion 3.1. Physico-chemical properties of Ir–V/CNFs and Ir–V/XC-72 HRTEM micrographs and histograms of the isolated Ir–V/CNFs and Ir–V/XC-72 are displayed in Fig. 1, which reveals that the XC-72 is dominantly amorphous carbon (Fig. 1b), while the graphene layers of CNFs are vertical to the fiber axis (Fig. 1a). Fig. 1 also gives that CNFs have an even diameter which is estimated to be 100 nm, but the diameter of XC-72 is about 30 nm. As also can be seen from Fig. 1, a good dispersion of metal particles was well identified on the surface of CNFs or XC-72, but the metal particle sizes are much different. The size of metal nanoparticles on Ir–V/CNFs is about 1 nm (Fig. 1a and a0 ), evidently smaller than that of Ir–V/XC-72 (Fig. 1b and b0 ). XC-72 is mainly occupied by amorphism carbon, which has poor interaction with the supported metal nanoparticles. Whereas, the edge atoms on CNFs can promote the interaction of carbon support with the supported metal nanoparticles, then, the metal nanoparticles on CNF surface exhibit a smaller size. Chen and co-workers [15] had decorated Pt– Ru nanoparticles on CNT surface, and they confirmed the edge atoms on CNTs are one reason for the small metal particle size. XPS was used to determine the surface oxidation states of the catalysts. As most of the atoms in small particle clusters are surface atoms, the oxidation state measured as such would also reflect well the bulk oxidation state. To investigate the XPS peaks, Gaussian–Lorentizan mixed shapes by using computer deconvolutions are frequently used to analysis the XPS spectrum. Fig. 2 displays that the Ir4f signal of the two catalysts, which is consisted of two pairs of doublets. The most intense doublet is due to metallic Ir, and the second set of doublets can be assigned to the Ir(IV) chemical state as in IrO2 [16,17].

2.2. Preparation and modification of electrode Electrocatalyst was dispersed ultrasonically in a 0.5 wt.% Nafion solution to obtain a homogenous black suspension with a concentration of 5 mg/ml. A 10 ll solution was pipetted onto the surface of a 5 mm diameter glassy carbon (GC) electrode. Before surface modification, the GC electrode was polished with 0.3 lm and 0.05 lm alumina slurries, washed by acetone, ethanol and water, and then subjected to ultrasonic agitation for 5 min in ultrapure water. 2.3. Electrocatalyst characterization The morphologies of catalysts were characterized by high resolution transmission electron microscope (HRTEM, JEOL TEM 2010). X-ray photoelectron spectroscopy (XPS) was recorded with a VG Escalab 200R spectrometer. The spectra were collected at a pass energy of 20 eV, which is typical for high-resolution conditions. Electrochemical measurements were performed with a CHI 730C electrochemical workstation (CHI Instrument, Inc., USA) in a 0.5 M HClO4 solution. The working electrode was GC coated with different electrocatalysts, and a saturated calomel reference electrode (SCE) was used for all electrochemical measurements, and all the potentials were reported versus this reference electrode. A Pt clump was used as the counter electrode.

Frequency, %

60

40

20

a' 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Particle Size/nm

Frequency, %

60

40

20

b' 0 1.0

1.5

2.0

2.5

3.0

3.5

Particle size/nm Fig. 1. HRTEM images of Ir–V/CNFs (a) and Ir–V/XC-72 (b) and particle size distribution of Ir–V/CNFs (a0 ) and Ir–V/XC-72 (b0 ).

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a

b Ir4f7/2

Ir4f 7/2 Ir4f 5/2

Intensity

Intensity

Ir4f5/2

60

65

55

70

60

Binding Energy/eV

65

70

Binding Energy/eV

4.0x10

-1

2.0x10

-1

a

Current density, mA/cm2

Current density, mA/cm2

Fig. 2. XPS spectra and their fitting curves of Ir–V/CNFs (a) and Ir–V/XC-72 (b).

1

0.0 -1

-2.0x10

-1

-4.0x10

0.30

2 0.45 0.60 0.75 0.90 Potential, (V vs. SCE)

4.0x10

-1

2.0x10

-1

b 1

0.0

-2.0x10

-1

-4.0x10

-1

2 0.30

0.45 0.60 0.75 0.90 Potential, (V vs. SCE)

Fig. 3. Cyclic voltammetric analysis in 0.5 M HClO4 solution saturated by nitrogen (1) and oxygen (2) at scan rate of 10 mV s1 of (a), Ir–V/CNFs and (b), Ir–V/XC-72.

Moreover, the Ir4f7/2 binding energy of Ir–V/CNFs is 61.6 eV, which is about 0.5 eV higher than that of Ir–V/XC-72 (61.1 eV). The slight shift in the Ir(0) peaks to higher binding energies is a known effect for small particles, as has been reported by Roth et al. [18]. On the other hand, the higher binding energy of Ir–V/ CNFs may be reflected the stronger metal-support interaction of metal particles with CNFs than those with XC-72. In a previous work, we had found that CNFs had higher binding energies with the supported Pt nanoparticles than other carbon materials [19]. 3.2. Effect of support microstructure on ORR properties Curve 1 and curve 2 in Fig. 3 are the current–potential curves of the electrolyte saturated by nitrogen and oxygen for Ir–V/CNFs and Ir–V/XC-72, respectively, which give the differences in ORR onset reduction potentials of Ir–V/CNFs and Ir–V/XC-72. Onset reduction potential is an indicator of the activity of surface reaction, and it is associated with the catalytic activity of catalyst [13]. ORR onset reduction potential of Ir–V/CNFs is about 0.53 V, while that of Ir– V/XC-72 is about 0.45 V, and the differences in onset reduction potential may be caused by the follow two reasons. Firstly, metal particles on CNFs have a smaller size than those on XC-72, and this may improve the catalytic performance. Takasu et al. [20] had investigated the size effect of Pt nanoparticles on ORR activity, and revealed that the exchange current density and the ORR activity increased when the Pt size decreased. Additionally, we also found Pt nanoparticles on CNFs have a size of about 1 nm are more active than those have an average size of 3 nm [21]. Secondly, CNFs have a stronger metal-support interaction that XC-72, and then may increase catalytic activity. In a previous study, we had found that Pt/CNFs had a higher ORR activity for the reason that CNFs

had a higher metal-support interaction with the supported Pt nanoparticles [19]. Roth et al. had also studied the influence of support microstructure on catalyst activity, and they manifested that the stronger metal-support could improve catalytic activity [18]. Furthermore, the oxygen reduction currents of the two electrodes are distinct, too. Table 1 lists the oxygen reduction current at the potential 0.30 V vs. SCE (at the kinetically controlled region) at the scan rate of 10 mV/s of the two electrocatalysts. The results indicate that the current density of Ir–V/CNFs is about one time larger than that of Ir–V/XC-72. One reason for the differences in the current densities of the two electrocatalysts is the smaller metal particle size on Ir–V/CNFs than that on Ir–V/XC-72. When the metal contents are the same, the smaller metal particles can provide more active sites. The other reason may be caused by the higher activity of Ir–V/CNFs compared with Ir–V/XC-72. ORR onset reduction potential of Ir–V/XC-72 is more negative than that of Ir–V/ CNFs, and then, the degree of surface activation of Ir–V/XC-72 is lower than that of Ir–V/CNFs at the same potential. At the kinetically controlled region, the current increases when the potential decreases for the increase in the degree of catalyst surface activation for a reduction reaction. In a previous study, we had revealed that the Pd/CNFs had a higher reduction current than Pd/activated carbon [13]. Similarly, Steigerwalt and co-workers [22] reported

Table 1 ORR onset reduction potential and current of different catalysts at the scan rate of 10 mV s1. Sample

Onset reduction potential (V)

Current density (mA cm2)

Ir–V/CNFs Ir–V/XC-72

0.53 0.45

0.267 0.125

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0.0 -4

4200

a

b

-1.0x10

ω / rap/min

100 200 400 1000 1600

-4

-3.0x10

-4

-4.0x10

-4

-5.0x10

3500 1/j [ A-1]

Current, A

-4

-2.0x10

2800

2100

-4

-6.0x10

-4

-7.0x10

0.0

0.2

0.4

0.6

0.8

1.0

0.02

Potential, (V vs. SCE)

0.04 0.06 0.08 0.10 1/2 -1/2 1/2 1/ω [ rad s ]

Fig. 4. Rotating disk electrode voltammetry curves for oxygen reduction on Ir–V/CNFs in O2 saturated 0.5 M HClO4 (a) and K–L plots for O2 reduction on Ir–V/CNFs at the potential of 0.00 V (b). The scan rate is 5 mV s1.

the Pt–Ru nanoparticles on CNFs performed a higher current density than those on XC-72. 3.3. ORR pathway of Ir–V/CNFs Fig. 4a displays the linear sweep voltammetric (LSV) results at rotation rates from 100 rap/min to 1600 rap/min for Ir–V/CNFs. Fig. 4a shows that above 0.50 V, a charge-transfer kinetic control is obtained. The mix kinetic diffusion control is obtained in the range of 0.50–0.30 V, and at more cathodic potential, the mass transfer-limited is become significant where a deference of the rotation rate is observed. When the rotation rate increased, limiting currents also increased due to the increase of the oxygen diffusion through the electrode surface. The over-all measured current density, j, of the oxygen reduction can be expressed as being dependent on the kinetic current density, jk , and the boundary-layer diffusion limited current density, jd . The over-all measured current density can generally be related by the Koutecky–Levich (K–L) equation. (1):

1 1 1 1 1  ¼ þ ¼ 2=3 1=6 b b j jk jd 0:62nFAD C O2 x1=2 nFAkC O2 O2 m

ð1Þ

where 0.62 is a constant, x is expressed in revolutions per minute, n is the number of electrons transferred per molecule of O2 reduced, F is the Faraday constant, C bO2 is the concentration of oxygen dissolved (1.1  106 mol cm3), DO2 is the diffusion coefficient of oxygen in

0.0000

Current, A

-0.0001 -0.0002 -0.0003

the solution (1.4  105 cm2 s1), and m the kinematic viscosity of the perchloric acid (1.0  102 cm2 s1) [23]. Molecular oxygen reduction involves the dissociative absorption of oxygen by a strong interaction between catalysts and oxygen, and then a redox reaction on the active site. The K–L plot of LSV results on Fig. 4b confirms that ORR on Ir–V/CNFs follows a multi-electron charge transfer (n = 4e) process to water formation. This is another advantage for Ir–V/CNFs as PEMFC cathode catalyst for the H2O2 formation not only will reduce the powder density of PEMFC, but also may wreck the structure of PEMFCs, such as accelerating the degradation of membrane, expediting the erosion of carbon support, etc. 3.4. Stability of Ir–V/CNFs electrode The stability of Ir–V/CNFs electrode was also examined as an oxygen cathode at 0.00 V in 0.5 M HClO4 solution for 14 h. From Fig. 5, it can be seen that no appreciable decay of the cell voltage was observed during the whole operation time, which manifests that the electrode has a high stability in the period of investigation. The highly stable response may come from the stability of electrode, stability of metal nanoparticle–CNFs interaction, and the high activity of Ir–V nanoparticles on CNF surfaces. 4. Conclusion Using chemical reduction, we had successfully deposited Ir–V nanoparticles on CNFs and Vulcan XC-72. Metal nanoparticles deposited on CNFs have been proven to possess smaller size than those on XC-72. Moreover, Ir–V/CNFs exhibits a higher ORR onset reduction potential than Ir–V/XC-72, which can be attributed to the smaller metal nanoparticles on CNFs as well as the strong interaction of metal nanoparticles with support. LSV analysis at different rotation rates shows that ORR on Ir–V/CNFs is a 4e reaction pathway. The results presented in this paper provide that Ir– V/CNFs can be an efficient electrocatalyst for oxygen reduction. Acknowledgements

-0.0004 0

15000

30000

45000

60000

Time /sec Fig. 5. Stability of the current originating from ORR on Ir–V/CNFs in 0.5 M HClO4 solution at 0.00 V.

The authors acknowledge the support from the International Sci. & Tech. Cooperation Program of Ministry of Science and Technology of China (Grant No. 2007DFC61690), the open-project program of the State Key Laboratory of Chemical Engineering (SKL-ChE-08C07), the China Postdoctoral Science Foundation (20080440645, 200902250), and the Science Foundation for Post doctors sponsored by the Science and Technology Committee of

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