Hollow carbon spheres with wide size distribution as anode catalyst support for direct methanol fuel cells

Hollow carbon spheres with wide size distribution as anode catalyst support for direct methanol fuel cells

Electrochemistry Communications 9 (2007) 1867–1872 www.elsevier.com/locate/elecom Hollow carbon spheres with wide size distribution as anode catalyst...

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Electrochemistry Communications 9 (2007) 1867–1872 www.elsevier.com/locate/elecom

Hollow carbon spheres with wide size distribution as anode catalyst support for direct methanol fuel cells Zhenhai Wen a

a,b

, Qiang Wang a, Qian Zhang a, Jinghong Li

a,*

Department of Chemistry, Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology, Tsinghua University, Beijing 100084, China b College of Chemistry and Chemical Engineering, Graduate University of Chinese Academy of Sciences, Beijing 100039, China Received 18 March 2007; received in revised form 18 April 2007; accepted 20 April 2007 Available online 30 April 2007

Abstract Hollow carbonaceous composites (HCCs) possessing sphere and hemisphere shape, which had wide size distribution between several tens of nanometers and several micrometers, were prepared through a facile hydrothermal method using glucose as carbon source with the assistance of sodium dodecyl sulfate (SDS). Pyrolysis of these hollow carbonaceous composites at 900 °C under nitrogen flow produced carbonized hollow carbon spheres (HCSs) without changing their structures. Platinum (Pt) was directly deposited on the surface of the HCSs by incipient wet method, using the NaBH4 as the reductant. TEM, SEM, powder XRD and FT-IR were utilized to characterize all these samples. It was found that Pt nanoparticles were uniformly anchored on the outer and the inner surface of HCSs. The electrocatalytic properties of the Pt/HCS electrode for methanol oxidation have been investigated through cyclic voltammetry and chronoamperometry. The Pt/HCS electrode showed significantly higher electrocatalytic activity and more stability for methanol oxidation compared with Pt supported carbon microspheres (Pt/CMs) and commercial carbon (Pt/XC-72) electrode. The excellent performance for the Pt/HCS might be attributed to the high dispersion of platinum catalysts and the particular hollow structure of HCSs. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Hollow carbon spheres; Catalyst support; Methanol electro-oxidation

1. Introduction Considerable attention has been paid to direct methanol fuel cells (DMFCs) due to their many advantages, including high energy density, the ease of handling, low operating temperature, and possible applications in transportation and portable electronic devices [1,2]. Electrocatalyst with higher activity is one of the key components in DMFC. Noble metals such as Pt and Pt–Ru are the most common catalysts in the anodic oxidation of methanol, but their high costs restrict their practical application. Therefore, it is highly desirable for researchers to develop economical and effective methods to obtain higher efficient methanol oxidation catalysts accompanying decrease in the quantity of noble metal, namely improve utilization efficiency of the *

Corresponding author. Tel./fax: +86 10 6279 5290. E-mail address: [email protected] (J. Li).

1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.04.016

catalysts. Usually, these electrocatalysts with nanostructure are dispersed over a high surface area and conductive supports, such as carbon black and carbon nanotubes [3–5]. It has been reported that the carbon supports play an important role in determining the morphologies and distributions of Pt nanoparticles, which in turn would influence the catalytic performance of the catalysts [6–9]. Therefore, it is very interesting to search novel carbon materials as supports in order to achieve a low loading of platinum with high catalytic performance. Hollow carbon spheres (HCSs) have inspired great interests due to their tailored structures, low density, high surface area, thermal insulation and electronic properties. As a result, HCSs have been investigated for large numbers of applications such as lithium batteries, gas energy storage, supports of catalytic, active-material encapsulation, drug delivery, adsorbents and lubricants [10–17]. For example, Hyeon et al. have recently reported the fabrication of

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hollow graphitic nanoparticles (30–40 nm) with high crystallinity through heat treatment of a mixture containing a polymeric and a metal salt. As catalytic support of DMFC electrodes, the hollow graphitic nanoparticles showed a higher current density and power density in comparison with commercial catalysts [18]. In this study, we reported a carbon material possessing hollow sphere and hemisphere structures with a wide size distribution ranging from several nanometers to several micrometers as the anode catalyst support of DMFC electrodes. It was found that the Pt nanoparticles supported on the hollow carbon spheres (HCSs) showed a higher catalytic activity for methanol electrochemical oxidation than those on both carbon microsphere and commercial carbon black Vulcan XC-72.

2.3. Characterization The morphologies of the samples were observed by using a Hitachi model H-800 Transmission Electron Microscope (TEM) and Scan Electron Micrographs (SEM, LEO 1530). Powder X-ray diffraction (XRD) was performed on a Bruker D8-Advance X-ray powder diffractometer with monochromatized CuKa radiation ˚ ), the data were collected between scattering (k = 1.5406 A angles (2h) of 10–70°. Fourier-transform IR (FT-IR) measurements were carried out through a Perkin–Elmer spectrophotometer operating in the infrared domain between 500 and 4000 cm 1 by using a KBr matrix. Specific surface areas were measured by Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption (Shimadzu, Micromeritics ASAP 2010 Instrument).

2. Experimental 2.4. Electrochemical measurements 2.1. Materials All the reagents were of analytical purity and were purchased from the Beijing Chemical Agent Company without purification, except Nafion emulsion (5 wt%, Dupont) were purchased from Sigma. For the purpose of comparison, carbon black powders (Vulcan XC-72, Cabot International) were also used without any further pretreatment. All solutions were prepared with Millipore-Q water (>18.0 MX cm). 2.2. Catalyst preparation Typically, 5.0 g glucose and 0.06 g SDS were dissolved in 25 ml water to form a homogeneous and clear solution under manual stirring. The mixture was then transferred to a 30 ml Teflon-lined stainless steel autoclave and heated at 170 °C for 10 h [10]. The resulted puce products were filtrated and washed with absolute ethanol and distilled water for several times. After drying at 60 °C, the HCSs were obtained through heat treatment of the HCCs at 900 °C under a nitrogen atmosphere for 2 h. For comparison, carbon microspheres (CMs) were prepared according to the method mentioned above, except that no SDS was added. The carbon-supported catalysts were synthesized at room temperature through a conventional reduction method by using H2PtCl6 Æ 6H2O as precursors of Pt catalysts and NaBH4 as the reductant. Typically, 0.05 g catalyst supports (HCSs, CMs, and XC-72) were added in 5.0 ml of 0.01 M H2PtCl6 solution and sonicated for 30 min to disperse them uniformly. Afterwards, 5 ml (0.08 M NaBH4 + 0.02 M NaOH) solution was slowly dropped into the mixtures under strong stirring. The mixtures were agitated vigorously for 10 h, and then the black solid slurries were centrifuged, washed and dried at 70 °C for 6 h in a vacuum oven. The three Pt supported carbons were nominated as Pt/HCSs, Pt/CMs and Pt/XC-72, respectively. The Pt loading of the catalysts was estimated as about 16.3 wt% based on the quantity of initial Pt source.

The catalyst performance for methanol oxidation reaction in room temperature was evaluated through cyclic voltammetry in the range of 0–1.0 V at a sweep rate of 50 mV/ s and chronoamperometry was measured at 0.6 V (versus Ag/AgCl) by a CHI 660 electrochemical workstation (CHI Inc., USA). To prepare the working electrode, a glass carbon (GC) electrode was firstly polished with a 0.3 and 0.05 lm alumina slurry, respectively, and washed with distilled water and acetone. The slurry containing 50 ll of 5.0 wt% Nafion, 1.0 ml of ethanol solution, and 5.0 mg of catalyst sample was ultrasonically dispersed for 30 min to form homogeneous ink, and then 5.0 ll aliquot of the catalytic slurry was homogeneously pipetted on the GC electrode surface. The electrodes after drying at room temperature were used as the working electrode. Electrochemical measurements were carried out in a three-electrode cell. Ag/AgCl electrode and Pt gauze were used as the reference and the counter electrodes, respectively. An aqueous solution of 0.5 M H2SO4 or 0.5 M CH3OH + 0.5 M H2SO4 was used for the electrolyte and the electrochemical experiments were carried out in nitrogen saturated solutions at room temperature. 3. Result and discussion TEM and SEM were utilized to characterize the morphologies of the as-prepared carbon materials. Fig. 1a and b show representative TEM and SEM images obtained from the HCCs without heat treatment, from which one can observe various opened hollow hemispheres and hollow microspheres. It was found that the hollow carbonaceous composites had wide size distribution in the range between several tens of nanometers and several micrometers. Interestingly, some small hollow spheres were sometimes embodied in the inner surface of the larger hollow hemisphere. As can be seen from Fig. 1c and d, the HCSs after heat treatment at 900 °C for 2 h under a flow of N2 almost remained the hollow structure of the carbonaceous

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Fig. 1. TEM images and SEM images of the HCCs (a, b) and the HCSs (c, d). Inset of (d) is a magnified SEM image of HCSs.

composites, where various opened hollow hemispheres and hollow microspheres can be observed too. TEM was further utilized to observe the morphology of the Pt/HCSs, as shown in Fig. 2. It could be observed that

Pt nanoparticles were well-dispersed on the surface of the HCSs in Fig. 2a. Fig. 2b and c shows typical TEM images of well-dispersed Pt nanoparticles supported on an open hollow hemisphere with about 1 lm and 300 nm diameters,

Fig. 2. TEM images of the Pt/HCSs (a–c), SAED of the Pt/HCSs (d), TEM images of Pt/CMs (e) and Pt/XC-72 (f).

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respectively. It could be observed that the Pt nanoparticles were anchored on both the inner and the outer surface of the hollow hemisphere. The inset of Fig. 2b exhibits a magnified TEM image obtained from the Pt/HCSs, where the Pt nanoparticles were anchored on the black edge of a hollow sphere uniformly. Selected area electron diffraction (SAED) patterns for the Pt/HCS exhibit several welldefined rings (Fig. 2d), indicating that the composites had a polycrystalline structure attributing to various diffraction planes of face-centered cubic (fcc) Platinum. For comparison, carbon microspheres (CMs) were also prepared through a similar method just no SDS was added during the hydrothermal process. Fig. 2e presents a typical TEM image of Pt/CMs, indicating that the CMs have a diameter of about 200 nm and their surface was covered by the Pt nanoparticles. Furthermore, commercial carbon materials (XC-72) were also used for the Pt catalyst supports. Fig. 2f shows a TEM image of the Pt/XC-72. However, apparent aggregations of Pt nanoparticles are seen in the Pt/XC-72 as embarked by arrow mark. The crystalline nature of the Pt nanoparticles anchoring on various carbon supports was recorded by X-ray powder diffraction (XRD) spectrum in the range of 10–70°. Fig. 3 shows the XRD patterns of the Pt/HCSs, Pt/CMs and Pt/XC-72, respectively. All of them show similar features with three characteristic diffraction peaks at 39.7°, 46.2° and 67.4° corresponding to the (1 1 1), (2 0 0), and (2 2 0) crystalline planes of fcc Pt, respectively, indicating that the Pt nanoparticles were composed of pure crystalline Pt. According to Scherrer formula, the average size of the Pt particles was estimated to be 5.7 nm for Pt/HCSs, 5.0 nm for Pt/CMs and 3.7 nm for Pt/XC-72 [19,20], which were in consistence with the result observed from TEM images. It is noted that a feeble peak positioned at about 25° was also observed, which could be corresponded to the (0 0 2) planes of carbonized carbon. As shown in the inset of Fig. 3, XRD patterns of HCSs without Pt loading, where two peaks assigned to (0 0 2) and (1 0 0) planes of graphite, can be observed at about 25° and 43.5°, indicating that the carbonized HCSs had graphite like crystallinity.

Fig. 3. XRD spectra of the Pt/HCSs (a), Pt/CMs (b) and Pt/XC-72 (c).

The electrochemical catalytic activities of the three carbon supported Pt (16.7 wt%) electrodes for the oxidation of methanol were investigated through cyclic voltammetry. Before performing cyclic voltammograms, the electrode was placed in the N2 saturated aqueous solution of 0.5 M H2SO4 containing 0.5 M CH3OH for 10 min to allow the system to reach a stable state. The stable CV curves of Pt/HCSs for methanol oxidation were obtained after about eight cycles and compared with Pt/CMs and Pt/XC-72 electrodes under the same condition in terms of the catalytic mass activity, as shown in Fig. 4 with dashed line, solid line, and thick solid line, respectively. All of the three electrodes show a similar feature, where one can observe a typical methanol oxidation current peak in the forward scan and an oxidation peak in the backward scan corresponding to the removal of the residual carbon species formed in the forward scan [21,22]. For the Pt/HCSs electrode, the mass current density initially rose above the background level at about 0.25 V (the ‘‘onset potential’’), which was more negative than that of both the Pt/CMs (at about 0.40 V) and the Pt/XC-72 (at about 0.45 V) electrodes. The more negative onset potential suggested that the Pt/HCSs had a positive effect on promoting the oxidation of methanol by lowering its overpotential. Additionally, the mass current density increased to 46.51 mA mg 1 at 0.68 V, which was about three times higher than that of the corresponding mass current density (14.70 mA mg 1) obtained on Pt/XC-72 electrode. The Pt/ CMs electrode presented a maximum current density of 20.28 mA mg 1, which was also much lower than that of Pt/HCSs electrode. It is well known that the ratio of the forward anodic peak current (If) to the backward anodic peak current (Ib) can be utilized to evaluate the catalyst tolerance to the intermediate carbonaceous species accumulated on the electrode surface [4,23,24]. The higher If/Ib value suggests that more methanol is completely oxidized to carbon dioxide. The ratios for the three kinds of carbon supported electrodes are calculated to be 1.28, 1.27 and 1.19,

Fig. 4. Cyclic voltamograms of the Pt/HCSs (a), Pt/CMs (b) and Pt/XC72 (c) in N2 saturated aqueous solution of 0.5 M H2SO4 + 0.5 M CH3OH at a scan rate of 50 mV/s.

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respectively, following the order: HCSs  CMs > Vulcan XC-72. The If/Ib value 1.28 of the Pt–CNTs–Pt was also higher than 0.88 of Pt/MWCNT reported previously [25], which further highlighted that the Pt/HCSs electrode possessed a higher catalytic efficiency for methanol oxidation than the XC-72 supported catalyst. In addition, the electrochemically active surface area (EASA) of Pt on the three carbon-base electrodes could also be evaluated through the hydrogen adsorption region of cyclic voltammograms in 0.5 M H2SO4 [18]. The EASA was estimated as 46.22 m2/g for Pt/HCSs, 29.41 m2/g for Pt/CMs and 24.71 m2/g for Pt/XC-72, respectively. The Pt/HCS also showed the highest EASA among them despite its largest size, possibly attributing to the highly dispersed Pt nanoparticles on unique HCSs that can advance the catalytic efficiency of Pt. The chronoamperometric technique is an effective method to evaluate the electrocatalytic activity and stability of electrode material. Fig. 5 shows typical current density–time responses for methanol oxidation measured at a fixed potential of 0.60 V on the three Pt supported carbon electrodes in 0.5 M H2SO4 aqueous solution containing 0.5 M CH3OH. All of them present a current decay during current–time measurements before a steady status was attained, which was attributed to the formation of some Pt oxide or adsorbed intermediates in methanol electro-oxidation reaction [20]. As expected, the methanol oxidation current at Pt/HCS electrode was evidently higher than that of Pt/CMs and Pt/XC-72 electrodes. In the steady-state region, the Pt/HCSs electrode presented the highest steady-state current density (1.01 mA mg 1) of methanol electro-oxidation after 600 s. While the Pt/CMs and PtVulcan XC-72R exhibited only an oxidation current density of about 0.15 mA mg 1. These results indicated that the Pt/HCSs had a higher activity and better stability than the latter two. According to the above results, the Pt/HCSs show the best catalytic performance for methanol oxidation among

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the three carbon materials. The reasons may be as follows: (1) The hollow structures of the HCSs play an important role in promoting the catalytic performance of methanol electro-oxidation when using HCSs as catalyst support. According to BET measurement for the three carbon materials, the surface areas of the three carbon materials were ranked in the following order: HCSs (255.5 m2/g) > MCs (225.3 m2/g) > XC-72 (216.1 m2/g). The higher specific surface area would be responsible to provide more activity sites, which was reconfirmed by the higher electrochemically active surface area. (2) According to TEM and SEM images, the as-synthesized HCSs were composed of different sized hollow microspheres ranging from several tens of nm to several lm, which could provide a disparity of pressure in the electrode. Therefore, the liquid reactants might facilely diffuse onto the surface of the catalyst and thus reduce the liquid sealing effect greatly, which would help to enhance the active surface area for electrochemical reactions. (3) It was believed that high conductivity (ionic or electronic) could be obtained due to the hollow structures of HCSs and thus contributed to achieve higher current density of methanol oxidation. Additionally, welldispersed platinum nanoparticles on both the outer and the inner surfaces of HCSs induced more triple-phase boundaries that helped in increasing the efficiency of the catalysts. 4. Conclusion In the present study, we have demonstrated a hollow carbon possessing spherical and hemispherical shaped support Pt catalyst as effective electrode material for methanol electrochemical oxidation. Pt nanoparticles were uniformly distributed on HCSs through a wetness method using H2PtCl6 and NaBH4 as the Pt source and reductant, respectively. The fact that the HCSs provided a particular structure for anchoring Pt on the outer or the inner surface of the support was unique in the system. Electrocatalytic oxidation of methanol at the Pt supported hollow carbon spheres electrode showed significantly higher activity and stability in comparison with Pt supported carbon microspheres and Pt/XC-72 electrodes. The high-performance for the Pt/HCSs can be attributed to higher BET surface area, well-dispersed platinum nanoparticles, high conductivity (ionic or electronic), the reduction of the liquid sealing effect and the unique hollow structures. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 20435010, No. 20575032) and 863 Project (2006AA05Z123). References

Fig. 5. Current–time curves of the Pt/HCSs (a), Pt/CMs (b) and Pt/XC-72 (c) in N2 saturated aqueous solution of 0.5 M H2SO4 containing 0.5 M CH3OH at a fixed potential of 0.6 V.

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