Pd nanoparticles supported on ultrahigh surface area honeycomb-like carbon for alcohol electrooxidation

international journal of hydrogen energy 35 (2010) 3263–3269

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Pd nanoparticles supported on ultrahigh surface area honeycomb-like carbon for alcohol electrooxidation Zaoxue Yan, Guoqiang He, Guanghui Zhang, Hui Meng, Pei Kang Shen* State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China

article info

abstract

Article history:

The honeycomb-like porous carbon was prepared using glucose as carbon source and solid

Received 7 December 2009

core mesoporous shell (SCMS) silica as templates. The material was characterized by

Received in revised form

physical and electrochemical methods. The results showed that the honeycomb-like

9 January 2010

porous carbon was consisted of hollow porous carbon (HPC) which gave an ultrahigh BET

Accepted 10 January 2010

surface area of 1012.97 m2 g1 and pore volume of 2.19 cm3 g1. The porous walls of the HPC

Available online 6 February 2010

were formed in the mesoporous shells of the silica templates. The HPC was used as the support to load Pd nanoparticles (Pd/HPC) for alcohol electrooxidation. It was highly active

Keywords:

for methanol, ethanol and isopropanol electrooxidation. The peak current density for

Porous carbon

ethanol electrooxidation on Pd/HPC electrode was five times higher than that on Pd/C

Mesopore

electrode at the same Pd loadings. The mass activity for ethanol electrooxidation was

Fuel cell

4000 A g1 which is much higher compared to the data reported in the literature. The highly

Silica template

porous structure of such HPC can be widely used as support for uniform dispersing metal

Ethanol electrooxidation

nanoparticles to increase their utilization as electrocatalysts.

Pd electrocatalyst

1.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

Introduction

Fuel cell technology has been developed very quickly since being pushed by spacecraft engineering in 1960s. However, the high cost has been remaining the choke point in the popularization of fuel cells, especially the high cost of noble metal supported electrocatalyst, which is one of the most critical materials in fuel cell systems [1,2]. One solution to reduce the cost of electrocatalysts is to develop novel supporting materials to decrease the amount of noble metal such as Pt and Pd by improving the dispersion and particle size at reasonable catalytic activity. Carbon materials such as carbon microspheres [3], carbon nanowires [4], carbon nanotubes [5–7], coin-like carbon [8], carbon aerogel [9], hollow carbon spheres [10] and three-dimentional carbon matrix [11] have been prepared for energy storage [12–15] and catalyst

supports [16–19], because of their large specific surface area, porosity, larger pore volume, low weight, chemical stability, excellent electronic conductivity, and so on. The porous carbons showed an enhancement effect on the activity of electrocatalysts in fuel cells [20–23]. There are many ways to prepare porous carbons including non-template [24–26] and template methods [27,28]. Template method is a conventional way to obtain porous carbon for controllable, ordered structure. Specially, silica template method has been developed extensively due to low cost and simple [29–31]. Recently, Pd-based catalysts have aroused much attention because they can be highly active for the alcohol oxidation in alkaline media where many non-noble metals are stable for electrochemical applications [32,33]. The mechanistic study on the electrooxidation of ethanol on Pd has also been reported based on an in situ FTIR spectroelectrochemical

* Corresponding author. Tel.: þ86 20 84036736; fax: þ86 20 84113369. E-mail address: [email protected] (P.K. Shen). 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.01.031

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international journal of hydrogen energy 35 (2010) 3263–3269

study of the ethanol oxidation on Pd in alkaline solution in which particular attention has been devoted to the effect of the pH [34]. Here, we report a novel method to prepare honeycomb-like porous carbon using glucose as carbon source and solid core mesoporous shell (SCMS) silica as templates. The honeycomb-like porous carbon is consisted by hollow porous carbon (HPC) which shows an ultrahigh BET surface area and pore volume. The Pd nanoparticles supported on such HPC as electrocatalysts were prepared for alcohol oxidation.

2.

Experimental

Solid core mesoporous shell (SCMS) silica was used as template for the preparation of honeycomb-like carbon [23]. The synthesis procedure of silica templates with a diameter of 70 nm was as follows. The tetraethoxysilane (TEOS, 10 ml, Guangzhou Chemical Reagent Co., China) was added into a solution containing ethanol (30 ml, Tianjin Fuyu Fine Chemical Co., Ltd, China), distilled-deinoized water (10 ml) and ammonia (30 wt.%, 2 ml, Guangdong Guanghua Chemical Co., Ltd, China) at 303 K with vigorous stirring. The mixture was stirred continuously for 1 h and dried overnight at 363 K to form silica templates. To prepare HPC, about 1.0 g silica templates, 0.5 g glucose (Tianjin Damao Chemical Reagent Co., China) and 50 ml distilled-deionized water were mixed in a flask and stirred dramatically with a magnetic stirrer in a water bath at 343 K until all the solvent was evaporated. Subsequently, the dried mixture was heated at 5 K/min up to 1123 K and held at that temperature for 3 h in nitrogen atmosphere for carbonization. The silica templates were finally removed by etching the products in 10% HF (Guangzhou Chemical Reagent Co., China) to generate porous carbon. The synthesis procedure is schematically illustrated in Scheme 1. Pd supported on HPC (denotes as Pd/HPC) or Vulcan XC-72 carbon (denotes as Pd/C) was prepared and used as electrocatalyst for alcohol oxidation. PdCl2 in glycol solution (0.94 ml, 0.1 mol dm3) was mixed with 50 mg HPC or Vulcan XC-72 carbon (Carbot. Co., USA) and treated in ultrasonic bath for 30 min to form uniform ink. The pH of the mixture was adjusted to 10 by 5 wt% NaOH/glycol solution. The ink was put into a homemade microwave oven (1000 W, 2.45 GHz) for alternative heating at a 10 s on and 10 s off procedure for 10 times. Afterwards, the mixture was washed with water for 4–5 times and dried in vacuum at 353 K for 2 h. For electrode preparation, Pd/HPC or Pd/C (5 mg) was dispersed in ethanol (1 ml) and Nafion suspension (1 ml, 0.5

wt%, DuPont, USA) under ultrasonic stirring to form the electrocatalyst ink. The electrocatalyst ink (40 ml) was then deposited on the surface of the glassy carbon rod (4 mm in diameter) and dried at room temperature overnight. The total Pd loadings were controlled at 0.02 mg cm2. All electrochemical measurements were performed in a three-electrode cell on a potentiastat (IM6e, Zahner-Electrik, Germany) at 303 K controlled by a water-bath thermostat. A platinum foil (3.0 cm2) and Hg/HgO were used as counter and reference electrodes, respectively. All chemicals were of analytical grade and used as received. The morphologies and size of the templates, Pd/HPC and Pd/C electrocatalysts were characterized by scanning electron microscopy (SEM, LEO 1530VP, Germany) and transmission electron microscopy (TEM, JOEP JEM-2010, JEOL Ltd.) operating at 200 kV. The structure of HPC and electrocatalysts were determined on a X-ray diffractometer (D/Max-IIIA, RigakuCo., ˚ radiation). The BET surface area, Japan,CuK1, l ¼ 1.54056 A pore volume and pore diameter were determined on a Physical Adsorption Instrument (ASAP 2400, Micrometeritics Co., USA).

3.

Results and discussion

Fig. 1 shows the SEM micrograms of the silica templates and the HPC. The honeycomb-like structure of the carbon materials with the pore size of about 70 nm is clear. The connection between pores after the removal of the templates resulted in a three-dimensional network which is expected to provide ultrahigh surface area. Fig. 2 shows the TEM images of the HPC with the threedimensional structure. The enlarged HRTEM image (Fig. 2b) of the HPC indicated that the walls were porously structured. The formation of the porous walls can be expressed as shown in Scheme 2. The glucose was melted and flowed down along the mesopores shell of the templates during the heating process. The porous walls formed after the removal of the templates. This mechanism is different from the formation of hollow carbon spheres as reported by Fu and co-workers [10]. The BET surface area and pore volume of the HPC were measured and summarized in Table 1. The surface area was as high as 1012.97 m2 g1. The pore size distribution of the HPC is shown in Fig. 3. It can be seen that the pore size distribution was consistent with the result as shown in Fig. 1c, indicating that the HPC was consisted of mesopores and macropores. The Pd nanoparticles loaded on such ultrahigh surface area HPC were used as electrocatalysts for alcohol oxidation. The Pd distribution on HPC is shown in Fig. 4a. Based on 100 Pd nanoparticles randomly selected, the average particle size

Scheme 1 – Schematic illustration of the synthesis procedure of the HPC.

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Fig. 2 – (a) TEM image of the HPC (bar 100 nm) and (b) HRTEM image of the HPC (bar 10 nm).

was calculated to be 7.1 nm. The corresponding histogram (Fig. 4b) indicates a Gaussian distribution of the Pd nanoparticles. Fig. 5 shows the XRD pattern of the Pd/HPC electrocatalyst. The diffraction peak observed at 2q of 24.8 corresponds to

Fig. 1 – SEM micrograms of (a) silica templates (bar 100 nm), (b) HPC (bar 2 mm) and (c) enlarged SEM microgram of HPC (bar 100 nm). Scheme 2 – Schematic diagram of the porous walls of hollow carbon spheres.

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Table 1 – The BET data of the HPC. Total surface area (m2 g1)

Micropore area (m2 g1)

Total pore volume (cm3 g1)

Micropore volume (cm3 g1)

1012.97

609.41

2.19

0.28

(002) facet of the hexagonal graphite, indicating a slightly graphitized amorphous carbon structure. The diffraction peaks observed at 2q of 39.8 , 46.1 and 67.6 correspond to the (111), (200) and (220) facets of the face-centered cubic structure of palladium crystal. The Pd (220) peak was used to calculate the particle size according to the Scherrer’s equation. D ¼ Kl=ðB cos qÞ where D presents the average diameter in nm, K presents the Scherrer constant (0.89), l presents the wavelength of X-ray (l ¼ 0.154056 nm), B presents the corresponding full width at half maximum (FWHM) of the (220) diffraction peak, and q presents the Bragg’s diffraction angle. The Pd particle size calculated is 6.7 nm which is very close to the TEM result. Fig. 6a shows the cyclic voltammograms of methanol, ethanol, and isopropanol oxidation on Pd/HPC electrodes. The Pd/HPC is active for all three alcohols. However, ethanol oxidation gave the best performance in terms of the onset potential and the peak current density comparing to other two alcohols. Fig. 6b compares the performance of ethanol oxidation on Pd/HPC and Pd/C electrodes at the same Pd loadings. The Pd/HPC shows five times higher peak current density than that of Pd/C for ethanol oxidation. It is reasonable by comparing the electrochemical active surface areas of both electrodes as shown in Fig. 6c. The highly porous structure of the HPC as support is beneficial for the uniform dispersion of the Pd nanoparticles to make them highly used and for the easier mass transfer. The deflection from the linear line of the data at lower scan rates in the relationship of peak current density and square root of scan rate on Pd/HPC electrode indicates the improvement in the mass transfer (see Fig. 6d). The straight line appears over the scan rate of 50 mV s1 on Pd/ HPC. On the other hand, it shows a linear line at any scan rate on Pd/C electrode, indicating the concentration polarization.

Fig. 4 – (a) TEM image of Pd/HPC electrocatalyst (bar 20 nm) and (b) corresponding particle size distribution of Pd/HPC.

1.5 1.0

Pd (200)

C (002)

2.0

Pd (220)

Pd (111)

2.5

Intensity

Pore volume / cm3 g-1

3.0

0.5 0.0 0

20

40

60

80

100

Pore diameter / nm Fig. 3 – Pore size distribution of the HPC.

20

30

40 50 60 2 theta / Degree

70

Fig. 5 – XRD pattern of Pd supported on HPC.

80

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a

80

Current density / mA cm-2

Current density / mA cm-2

international journal of hydrogen energy 35 (2010) 3263–3269

ethanol methanol isopropanol

60 40 20 0 -0.8

-0.6

-0.4

-0.2

0.0

b

80 60

Pd/HPC

40

Pd/C

20 0 -0.8

0.2

-0.6

c

Current density / mA·cm-2

Current density / mA cm-2

2

Pd/HPC

1

Pd/C 0 -1 -2 -3 -0.8

-0.4

-0.2

0.0

0.2

E / V vs. Hg/HgO

E / V vs. Hg/HgO 120

d Pd/HPC

100 80 60 40

Pd/C

20 0

-0.6

-0.4

-0.2

0.0

0.2

2

0.4

4

6

8

10

12

14

16

18

v1/2/ (mV s-1)1/2

E / V vs. Hg/HgO

Fig. 6 – (a) Cyclic voltammograms of different alcohols oxidation on Pd/HPC electrode in 1.0 mol dmL3 KOH/1.0 mol dmL3 alcohol solution at 303 K, scan rate: 50 mV sL1, (b) cyclic voltammograms of ethanol oxidation on Pd/C and Pd/HPC electrodes in 1.0 mol dmL3 KOH/1.0 mol dmL3 ethanol solution at 303 K, scan rate: 50 mV sL1, (c) cyclic voltammograms of Pd/C and Pd/HPC in 1.0 mol dmL3 KOH solution at 303 K, scan rate: 50 mV sL1 and (d) plots of the peak current density against the square root of the scan rate for both electrodes.

The effect of the ethanol concentration on the performance of the Pd/HPC electrode was also performed. The results indicated that the ethanol oxidation in the solution

0.3

Pd/HPC 8 mA cm-2 Pd/HPC 16 mA cm-2 Pd/HPC 20 mA cm-2

E / V vs. Hg/HgO

0.2

Pd/HPC 28 mA cm-2 Pd/C 1.2 mA cm-2

0.1 0.0

containing 1 mol dm3 ethanol presented the best performance (Data are not shown). The chronopotentiometric testing further proved that the Pd/HPC electrocatalyst could sustain larger current densities for stable ethanol oxidation than that of Pd/C electrocatalyst as shown in Fig. 7. The electrode potential was polarized to higher potentials at higher current densities than 8 mA cm2 on Pd/HPC due to the loss of the catalytic activity for ethanol oxidation. However, the Pd/C electrode could not sustain the constant current density of 1.2 mA cm2 due to the lower active surface area or lower utilization of Pd even at the same Pd loadings.

-0.1 -0.2

4.

-0.3

The honeycomb-like porous carbon with ultrahigh BET surface area and macropores and mesopores was prepared using glucose as carbon source and solid core mesoporous shell (SCMS) silica as templates. The honeycomb-like porous carbon was consisted of hollow porous carbon (HPC) which showed an ultrahigh BET surface area of 1012.97 m2 g1 and pore volume of 2.19 cm3 g1. The porous walls of the HPC were formed in the mesoporous shells of the templates with different mechanism compared to the hollow carbons reported in the literature [10,35]. The distribution of the pore size

-0.4 0

10

20

30

40

50

60

t / min Fig. 7 – The chronopotentiometric curves of ethanol oxidation on Pd/C and Pd/HPC at different current densities in 1.0 mol dmL3 ethanol/1.0 mol dmL3 KOH solution, 303 K.

Conclusions

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international journal of hydrogen energy 35 (2010) 3263–3269

peaked at 55 nm which around the size of the templates. The highly porous structure of the HPC as support is beneficial for the uniform dispersion of the metal nanoparticles to increase their utilization. The HPC was used as the support of the Pd nanoparticles (Pd/HPC) for alcohol electrooxidation. It was highly active for methanol, ethanol and isopropanol electrooxidation. The peak current densities for ethanol electrooxidation on Pd/HPC electrode were five times higher than that on Pd/C electrode at the same Pd loadings due to the higher electrochemical active surface area of the Pd on highly porous carbon support. The mass activity for ethanol electrooxidation was 4000 A g1 (conversed from the datum in Fig. 6a) which is much higher compared to the data reported in the literature.

Acknowledgements The work was supported by the China National 863 Program (2009AA05Z110), the Guangdong Sci. & Tech. Key Projects (2007A010700001, 2007B090400032) and Guangzhou Sci. & Tech. Key Projects (2007Z1-D0051, SKT[2007]17-11).

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