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Microwave synthesis and characterization of Pt nanoparticles supported on undoped nanodiamond for methanol electrooxidation
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 7 ( 2 0 1 2 ) 1 2 2 0 e1 2 2 5
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Microwave synthesis and characterization of Pt nanoparticles supported on undoped nanodiamond for methanol electrooxidation L.Y. Bian a,b, Y.H. Wang a, J.B. Zang a,*, F.W. Meng a, Y.L. Zhao a a
State Key Laboratory of Metastable Material Science and Technology, College of Material Science and Engineering, Yanshan University, Qinhuangdao 066004, China b College of Physics and Chemistry, Henan Polytechnic University, Jiaozuo, Henan 454000, China
article info
abstract
Article history:
Platinum nanoparticles supported on undoped nanodiamond (ND) with an average particle
Received 21 July 2011
size of 50 nm were prepared using a microwave-heating polyol method. This method
Received in revised form
involves the addition of different amounts of chloroplatinic acid in the synthesis solution
17 September 2011
to obtain different Pt mass percentages. The Pt/ND catalysts were characterized by energy-
Accepted 27 September 2011
dispersive spectroscopy, transmission electron microscopy, and X-ray diffraction. The
Available online 19 October 2011
small and uniform Pt nanoparticles were highly dispersed on ND supports. The mean size of the Pt particles was 4e5 nm. The effect of Pt loading on catalytic performance was
Keywords:
investigated. Electrochemical measurements demonstrate that the Pt/ND catalysts
Undoped nanodiamond
prepared with proper Pt mass percentage exhibited a significantly high electrocatalytic
Microwave
activity for methanol electrooxidation.
Platinum
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Catalyst
1.
Introduction
A direct methanol fuel cell, which uses methanol directly without prior complex reforming, has attracted significant attention due to several advantages, such as easy transportation and storage of the fuel, reduced system size, weight, complexity, and high energy efficiency [1e6]. The importance of catalyst support for fuel cells has recently been recognized, and various forms of carbon, such as activated carbon, carbon black, carbon nanotubes (CNTs), and carbon nanofibers, have been suggested [7e15]. Structure, surface area, and porosity directly affect the performance of the catalysts. However, sp2bonded carbon is susceptible to corrosion and microstructural degradation during anodic polarization [16], leading to a loss of activity due to catalyst detachment. Moreover, this
susceptibility can lead to catastrophic electrode failure [16e21]. Thus, the development of advanced and more stable supporting materials is highly advantageous [16]. Undoped nanodiamond (ND) powder is a novel carbon material synthesized by detonation [22e24] or milling of synthetic microdiamonds [25]. ND powder not only has the properties of diamond, but also possesses the features of a nanoscale material, such as an ultrafine particle size and a giant specific surface area as well as large numbers of surface defects. Therefore, this powder can provide a large catalytic area similar to other sp2-bonded carbon nanomaterial. Moreover, ND powder, unlike sp2-bonded carbon nanomaterials, has excellent thermal and chemical stability. These properties facilitate the feasibility of the ND powder as a stable catalytic support material. In a previous work, the
* Corresponding author. Tel.: þ86 13930308747; fax: þ86 335 8387679. E-mail address: [email protected] (J.B. Zang). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.09.118
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 7 ( 2 0 1 2 ) 1 2 2 0 e1 2 2 5
Fig. 1 e XRD pattern of the Pt/ND catalyst.
authors coated platinum (Pt) nanoparticles on undoped ND powder with average particle sizes of 5 and 100 nm using electrodeposition from 1.1 mmol/L chloroplatinic acid. The Pt/ ND nanoparticles exhibited good electrocatalytic properties for methanol electrooxidation [26]. The microwave-assisted polyol process, a rapid and uniform heating method, has received considerable attention
1221
as a promising method to synthesize nanosized materials [27e32]. In this method, ethylene glycol (EG) is commonly used as a solvent and reducing agent for metal salt compounds as well as a protecting agent for metal nanoparticles. The polymer-stabilized Pt, Ru, Ag, and Pd colloids with small and uniform particle sizes have been prepared using the microwave-assisted polyol process [30,33e36]. In previous studies, Pt/XC-72 carbon [37], Pt/graphite nanofibers [38] and Pt/CNTs [39] were prepared using this process and showed very good electrocatalytic properties for methanol electrooxidation. In the present work, Pt was coated on the surface of ND powder by microwave-heating EG solutions of platinum precursor salt. The electrocatalytic effects of the catalysts with different Pt mass percentages on methanol oxidation were investigated.
2.
Experimental
ND powder with an average particle size of 50 nm was fabricated by mechanical crushing and was supplied by Henan Boreas Technological Development Co. A platinum precursor (H2PtCl6$6H2O), EG, and sulfuric acid (H2SO4) were purchased from Shanghai Chemical Products Ltd. Deionized water was
Fig. 2 e TEM images of (a) Pt/ND-a, (b) Pt/ND-b, (c) Pt/ND-c, and (d) Pt/ND-d.
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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 7 ( 2 0 1 2 ) 1 2 2 0 e1 2 2 5
used to prepare the solutions, and high-purity nitrogen gas was also used in the experiments. In a typical procedure, 2.0 mL aqueous H2PtCl6 solution (0.055 mol/L) was mixed with 25 mL EG in a 100 mL beaker. Then, the mixture was uniformly mixed with 80, 40, 20, or 10 mg of 50 nm ND using ultrasound to obtain different Pt mass percentages. The beaker was placed at the center of a microwave oven and was heated for 80 s at 800 W. The resulting suspension was filtered, and the residue was washed with acetone and deionized water. The solid products were dried at 100 C for 12 h in a vacuum oven. Pt/ND-a, Pt/ND-b, Pt/ ND-c, and Pt/ND-d samples, corresponding to the addition of 80, 40, 20, and 10 mg ND in the mixtures were obtained, respectively. Energy dispersion spectra (EDS) analysis was performed to determine the metal mass fraction of the Pt/ND catalysts. Xray diffraction (XRD) was employed to determine the crystalline structure of the samples. A JEM-2010 high-resolution transmission electron microscope (TEM) was used to characterize the morphology of the catalysts. Electrocatalytic activities of Pt/ND electrocatalysts for methanol electrooxidation were measured by cyclic voltammetry (CV) and chronoamperometry (CA) using a CHI660A electrochemical workstation. A conventional three-electrode system was used, consisting of a Pt/ND electrode as the working electrode, a platinum coil auxiliary electrode, and an Ag/AgCl electrode as the reference electrode. The experiments were conducted at 25 C. The working electrode was obtained as follows: Pt/ND catalysts (10 mg) were dispersed in 10 mL ethanol by ultrasonication for 30 min to achieve a 1.0 mg/mL suspension, and one drop of the suspension was directly cast on the surface of the glassy carbon electrode and evaporated at room temperature. The active specific surface area of the Pt particles was calculated according to a hydrogen electrosorption curve. The potential was cycled between þ1.0 and 0.3 V in 1.0 mol/L H2SO4 electrolyte at 50 mV/s to obtain the voltammograms of hydrogen adsorption. All electrolyte solutions were deaerated by high-purity nitrogen for 30 min prior to any electrochemical measurement.
3.
Results and discussion
3.1.
XRD characterizations of Pt/ND catalysts
d (d) catalysts prepared using the microwave polyol process with different Pt mass percentages. Table 1 lists the corresponding relations between the ratio of Pt to ND support in the starting mixture and the Pt mass percentage in Pt/ND electrocatalysts based on the EDS analysis. H2PtCl6 added in the starting mixture was suggested to be almost totally reduced. Fig. 2 shows that the Pt nanoparticles are dispersed on the facet surfaces of ND, and that these nanoparticles are approximately 4e5 nm and are uniformly distributed. Pt nanoparticles become denser on the surface of ND as the Pt mass percentage of the Pt/ND nanocomposite increases. When the Pt mass percentage is 50.1%e51.5% (Pt/ND-c), the ND surface is almost studded with Pt particles (Fig. 2c). Further increase in Pt mass percentage results in the onset of agglomeration of Pt particles (Fig. 2d), which could lead to a reduction on the surface area of Pt.
3.3.
Methanol electrooxidization over Pt/ND catalysts
Fig. 4 shows the CV curves of methanol electrooxidization by the prepared catalysts in 1.0 mol/L CH3OHþ0.5 mol/L H2SO4 solutions. Two peaks of methanol oxidation are observed at 0.64 and 0.43 V. The characteristics of the CV curves and peak potentials (Ep) are similar to those of other catalysts with sp2bonded carbon as support [40,41], proving the catalytic activity of the Pt/ND composite toward methanol oxidation. As shown in Fig. 4, Pt/ND-a displays the lowest peak current (curve a) for methanol electrooxidation among the Pt/ND catalysts. The
The structure of the Pt/ND composite was characterized by XRD (Fig. 1). The XRD measurement confirmed the presence of crystalline Pt nanoparticles with characteristic peaks at 2q values of 39.8 , 46.2 , 67.4 , and 81.2 , corresponding to the reflection planes (1 1 1), (2 0 0), (2 2 0), and (3 1 1), respectively. The peaks could be indexed to face-centered cubic platinum, and the widening of the peaks implies the nanoscale characteristic of the Pt particles. The most intense peaks appeared at 43.98 , 75.28 , and 91.68 , corresponding to the (1 1 1), (2 2 0), and (3 1 1) cubic diamond planes.
3.2.
TEM and EDS characterizations of Pt/ND catalysts
Figs. 2 and 3 show the TEM images and the corresponding EDS spectra of the Pt/ND-a (a), Pt/ND-b (b), Pt/ND-c (c), and Pt/ND-
Fig. 3 e EDS spectra of (a) Pt/ND-a, (b) Pt/ND-b, (c) Pt/ND-c, and (d) Pt/ND-d.
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 7 ( 2 0 1 2 ) 1 2 2 0 e1 2 2 5
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Table 1 e Comparison of calculated and measured values of Pt mass percentage. Samples
Pt/ND-a Pt/ND-b Pt/ND-c Pt/ND-d
Calculated ND H2PtCl6 Measured values mass (55 mmol/L) values of (mL) of Pt mass (mg) Pt mass percentage percentage (%) (%) (from EDS) 80 40 20 10
2 2 2 2
21.2 34.9 51.8 68.2
20.6e21.6 31.5e35.4 50.1e51.5 65.6e66.9
low electrocatalytic activity is due to the sparse distribution of the Pt particle on the ND surface (Fig. 2a). When the Pt mass percentage is increased, Pt/ND catalysts display higher peak currents for methanol electrooxidation. The highest oxidation current occurs on Pt/ND-c (curve c), whose surface presents the most uniform and dense Pt nanoparticles (Fig. 2c). However, with increasing Pt mass percentage (Pt/ND-d), peak currents begin to decrease significantly (curve d) because of the agglomeration of Pt particles on the surface (Fig. 2d). The CA profiles of methanol electrooxidation on Pt/ND electrodes at 0.6 V are displayed in Fig. 5. The Pt/ND-c catalysts (curve c) exhibit not only high initial current, but also a higher current compared with those of the other prepared catalysts (curve a, b, and d). This characteristic indicates that catalysts with 50.1%e51.5% Pt mass percentages yield better performance. The currents of methanol electrooxidation on the Pt/C catalysts decrease moderately with time (Fig. 5). The current decay is due to the intermediate products of methanol oxidization such as CO and other ions in the electrolyte adsorbed onto the Pt nanoparticles that inhibit methanol electrooxidization. The Pt/ND-d catalysts (curve d in Fig. 5) exhibited not only lower currents compared with Pt/ND-c (curve c in Fig. 5), but also a faster decrease in speed compared with that of Pt/ND-b (curve b in Fig. 5). These results indicate that the agglomeration of Pt particles on the ND surface leads to an unstable electrocatalytic activity.
Fig. 4 e CV curves of methanol electrooxidization on (a) Pt/ ND-a, (b) Pt/ND-b, (c) Pt/ND-c, (d) Pt/ND-d in 1.0 mol/L CH3OHD0.5 mol/L H2SO4.
Fig. 5 e CA curves of (a) Pt/ND100-a, (b) Pt/ND100-b, (c) Pt/ ND100-c, and (d) Pt/ND100-d electrodes in 1.0 mol/L CH3OHD0.5 mol/L H2SO4.
3.4.
Active specific surface area of Pt/ND catalysts
The actual surface area of Pt particles is generally one of the important parameters in determining the catalytic properties of catalysts for methanol electrooxidation considering that this reaction is surface sensitive. The active specific surface area of Pt particles for Pt/ND-c catalysts can be estimated from the integrated charge in the hydrogen adsorption region of the CV curves (hatched area of Fig. 6). The areas (m2/g) were calculated from the following formula, assuming a correspondence value of 0.21 mC/cm2, a value generally used for polycrystalline Pt electrodes [42], and Pt loading: AEL m2 =g Pt ¼ QH = 0:21 103 C ðg ptÞ
(1)
where AEL is the active specific surface area of Pt particles obtained electrochemically, QH is the amount of charge exchanged during the electro-adsorption of hydrogen atoms
Fig. 6 e Hydrogen electro-adsorption CV curve for Pt/ ND100-c in 1.0 mol/L H2SO4, v [ 0.05 V/s.
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on Pt, and C is the unit Coulomb. The obtained AEL of the Pt/ ND-c catalysts was 64.5 m2/g Pt, corresponding approximately to that of Pt/C catalysts with Pt nanoparticles of the same size [43].
4.
Conclusions
Pt/ND catalysts with different Pt mass percentages were synthesized using a microwave-assisted polyol method. The mean size of the Pt particles was 4e5 nm. Pt/ND catalysts with 50.1%e51.5% Pt mass percentages exhibited better electrocatalytic performance for methanol oxidation. The improvement in electrocatalytic performance was because the fact that the prepared Pt/ND catalyst had small, uniform, dense, and highly dispersed Pt nanoparticles when a proper amount of chloroplatinic acid was added to the synthesis solution. However, further increase in Pt mass percentage resulted in the agglomeration Pt particles on the ND surface, leading to a worse catalytic performance. The microwave-assisted polyol method for preparing Pt/ ND catalysts in the present paper is simple, effective, and can be used as an alternative process for preparing highperformance electrocatalysts.
Acknowledgements This work was funded by the National Natural Science Foundation of China (Grant Nos. 50872119 and 50972125) and the Natural Science Foundation of Hebei Province (Grant Nos. E2011203126 and E2010001187). The authors also gratefully acknowledge the funding from Element Six Co. J.B. Zang thank the China Postdoctoral Science Foundation (Grant No. 20100480197) as well for its assistance.
references
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[7] Chai GS, Yoon SB, Yu JS, Choi JH, Sung YE. Ordered porous carbons with tunable pore sizes as catalyst supports in direct methanol fuel cell. J Phys Chem B 2004;108:7074e9. [8] He Z, Chen J, Liu D, Zhou H, Kuang Y. Electrodeposition of PteRu nanoparticles on carbon nanotubes and their electrocatalytic properties for methanol electrooxidation. Diamond Relat Mater 2004;13:1764e70. [9] Guo J, Sun G, Wang Q, Wang G, Zhou Z, Tang S, et al. Carbon nanofibers supported Pt-Ru electrocatalysts for direct methanol fuel cells. Carbon 2006;44:152e7. [10] Hsieh C-T, Hung W-M, Chen W-Y. Electrochemical activity and stability of Pt catalysts on carbon nanotube/carbon paper composite electrodes. Int J Hydrogen Energ 2010;35: 8425e32. [11] Wang ZB, Zuo PJ, Chu YY, Shao YY, Yin GP. Durability studies on performance degradation of Pt/C catalysts of proton exchange membrane fuel cell. Int J Hydrogen Energ 2009;34: 4387e94. [12] Ahmadi R, Amini MK. Synthesis and characterization of Pt nanoparticles on sulfur-modified carbon nanotubes for methanol oxidation. Int J Hydrogen Energ 2011;36:7275e83. [13] Zhiani M, Rezaei B, Jalili J. Methanol electro-oxidation on Pt/C modified by polyaniline nanofibers for DMFC applications. Int J Hydrogen Energ 2010;35:9298e305. [14] Jung DH, Bae SJ, Kim SJ, Nahm KS, Kim P. Effect of the Pt precursor on the morphology and catalytic performance of Pt-impregnated on Pd/C for the oxygen reduction reaction in polymer electrolyte fuel cells. Int J Hydrogen Energ 2011;36: 9115e22. [15] Sieben JM, Duarte MME, Mayer CE. Methanol oxidation on carbon supported Pt-Ru catalysts prepared by electrodeposition-Evaluation of Nafion 117 film effect. Int J Hydrogen Energ 2010;35:2018e24. [16] Hupert M, Muck A, Wang J, Stotter J, Cvackova Z, Haymond S, et al. Conductive diamond thin-films in electrochemistry. Diamond Relat Mater 2003;12:1940e9. [17] Paik CH, Jarvi TD, O’Grady WE. Extent of PEMFC cathode surface oxidation by oxygen and water measured by CV. Electrochem Solid-State Lett 2004;7. A82eAA4. [18] Kangasniemi KH, Condit DA, Jarvi TD. Characterization of Vulcan electrochemically oxidized under simulated PEM fuel cell conditions. J Electrochem Soc 2004;151. E125eEE32. [19] Wang J, Swain GM, Tachibana T, Kobashi K. Incorporation of Pt particles in boron-doped diamond thin films applications in electrocatalysis. Electrochem Solid-State Lett 2000;3: 286e9. [20] Wang J, Swain GM. Fabrication and evaluation of platinum/ diamond composite electrodes for electrocatalysis. J Electrochem Soc 2003;150:E24e32. [21] Antolini E. Formation, microstructural characteristics and stability of carbon supported platinum catalysts for low temperature fuel cells. J Mater Sci 2003;38:2995e3005. [22] Mironov E, Koretz A, Petrov E. Detonation synthesis ultradispersed diamond structural properties investigation by infrared absorption. Diamond Relat Mater 2002;11:872e6. [23] Iakoubovskii K, Baidakova MV, Wouters BH, Stesmans A, Adriaenssens GJ, Vul’ AY, et al. Structure and defects of detonation synthesis nanodiamond. Diamond Relat Mater 2000;9:861e5. [24] Shenderova OA, Zhirnov VV, Brenner DW. Carbon nanostructures. Crit Rev Solid State Mater Sci 2002;27: 227e356. [25] Krueger A. Diamond nanoparticles: jewels for chemistry and physics. Adv Mater 2008;20:2445e9. [26] Bian LY, Wang YH, Zang JB, Yu JK, Huang H. Electrodeposition of Pt nanoparticles on undoped nanodiamond powder for methanol oxidation electrocatalysts. J Electroanal Chem 2010;644:85e8.
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[27] Tsuji M, Hashimoto M, Nishizawa Y, Kubokawa M, Tsuji T. Microwave-assisted synthsis of metallic nanostructures in solution. Chem Eur J 2005;11:440e52. [28] Miyazaki A, Balint I, Aika K, Nakano Y. Preparation of Ru nanoparticles supported on g-Al2O3 and its novel catalytic activity for ammonia synthesis. J Catal 2001;204: 364e71. [29] Liao XH, Zhu JM, Zhu JJ, Xu JZ, Chen HY. Preparation of monodispersed nanocrystalline CeO2 powders by microwave irradiation. Chem Commun; 2001:937e8. [30] Komarneni S, Li D, Newalkar B, Katsuki H, Bhalla AS. Microwave-polyol process for Pt and Ag nanoparticles. Langmuir 2002;18:5959e62. [31] Katsuki H, Komarneni S. Microwave-hydrothermal synthesis of monodispersed nanophase a-Fe2O3. J Am Ceram Soc 2001; 84:2313e7. [32] Zhu JJ, Palchik O, Chen SG, Gedanken A. Microwave assisted preparation of CdSe, PbSe, and Cu2-xSe nanoparticles. J Phys Chem B 2000;104:7344e7. [33] Yu W, Tu W, Liu H. Synthesis of nanoscale platinum colloids by microwave dielectric heating. Langmuir 1999; 15:6e9. [34] Tu WX, Lin HF. Continuous synthesis of colloidal metal nanoclusters by microwave irradiation. Chem Mater 2000;12: 564e7. [35] Yan XP, Liu HF, Liew KY. Size control of polymer-stabilized ruthenium nanoparticles by poyol reduction. J Mater Chem 2001;11:3387e91.
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[36] Hsieh C-T, Hung W-M, Chen W-Y, Lin J-Y. Microwaveassisted polyol synthesis of Pt-Zn electrocatalysts on carbon nanotube electrodes for methanol oxidation. Int J Hydrogen Energ 2011;36:2765e72. [37] Chen WX, Lee JY, Liu ZL. Microwave-assisted synthesis of carbon supported Pt nanoparticles for fuel cell application. Chem Commun 2002;0:2588e9. [38] Zhao J, Chen WX, Han G, Zheng YF, Xu ZD. Platinum nanoparticles supported on graphite nanofibers prepared by microwave irradiation and its electrocatalytic activity. Chin Chem Lett 2005;16:269e72. [39] Chen WX, Zhao J, Lee JY, Liu ZL. Microwave heated polyol synthesis of carbon nanotubes supported Pt nanoparticles for methanol electrooxidation. Mater Chem Phys 2005;91:124e9. [40] Shen PK, Tian Z. Performance of highly dispersed Pt/C catalysts for low temperature fuel cells. Electrochim Acta 2004;49:3107e11. [41] Fu Z, Li W, Zhang W, Sun F, Zhou Z, Xiang X. Preparation and activity of carbon-supported porous platinum as electrocatalyst for methanol oxidation. Int J Hydrogen Energ 2010;35:8101e5. [42] Liu ZL, Lee JY, Han M, Chen WX, Gan LM. Synthesis and characterization of PtRu/C catalysts from microemulsions and emulsions. J Mater Chem 2002;12:2453e8. [43] Zhao J, Wang P, Chen W, Liu R, Li X, Nie Q. Microwave synthesis and characterization of acetate-stabilized Pt nanoparticles supported on carbon for methanol electrooxidation. J Power Sources 2006;160:563e9.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Microwave synthesis and characterization of Pt nanoparticles supported on undoped nanodiamond for methanol electrooxidation L.Y. Bian a,b, Y.H. Wang a, J.B. Zang a,*, F.W. Meng a, Y.L. Zhao a a
State Key Laboratory of Metastable Material Science and Technology, College of Material Science and Engineering, Yanshan University, Qinhuangdao 066004, China b College of Physics and Chemistry, Henan Polytechnic University, Jiaozuo, Henan 454000, China
article info
abstract
Article history:
Platinum nanoparticles supported on undoped nanodiamond (ND) with an average particle
Received 21 July 2011
size of 50 nm were prepared using a microwave-heating polyol method. This method
Received in revised form
involves the addition of different amounts of chloroplatinic acid in the synthesis solution
17 September 2011
to obtain different Pt mass percentages. The Pt/ND catalysts were characterized by energy-
Accepted 27 September 2011
dispersive spectroscopy, transmission electron microscopy, and X-ray diffraction. The
Available online 19 October 2011
small and uniform Pt nanoparticles were highly dispersed on ND supports. The mean size of the Pt particles was 4e5 nm. The effect of Pt loading on catalytic performance was
Keywords:
investigated. Electrochemical measurements demonstrate that the Pt/ND catalysts
Undoped nanodiamond
prepared with proper Pt mass percentage exhibited a significantly high electrocatalytic
Microwave
activity for methanol electrooxidation.
Platinum
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Catalyst
1.
Introduction
A direct methanol fuel cell, which uses methanol directly without prior complex reforming, has attracted significant attention due to several advantages, such as easy transportation and storage of the fuel, reduced system size, weight, complexity, and high energy efficiency [1e6]. The importance of catalyst support for fuel cells has recently been recognized, and various forms of carbon, such as activated carbon, carbon black, carbon nanotubes (CNTs), and carbon nanofibers, have been suggested [7e15]. Structure, surface area, and porosity directly affect the performance of the catalysts. However, sp2bonded carbon is susceptible to corrosion and microstructural degradation during anodic polarization [16], leading to a loss of activity due to catalyst detachment. Moreover, this
susceptibility can lead to catastrophic electrode failure [16e21]. Thus, the development of advanced and more stable supporting materials is highly advantageous [16]. Undoped nanodiamond (ND) powder is a novel carbon material synthesized by detonation [22e24] or milling of synthetic microdiamonds [25]. ND powder not only has the properties of diamond, but also possesses the features of a nanoscale material, such as an ultrafine particle size and a giant specific surface area as well as large numbers of surface defects. Therefore, this powder can provide a large catalytic area similar to other sp2-bonded carbon nanomaterial. Moreover, ND powder, unlike sp2-bonded carbon nanomaterials, has excellent thermal and chemical stability. These properties facilitate the feasibility of the ND powder as a stable catalytic support material. In a previous work, the
* Corresponding author. Tel.: þ86 13930308747; fax: þ86 335 8387679. E-mail address: [email protected] (J.B. Zang). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.09.118
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 7 ( 2 0 1 2 ) 1 2 2 0 e1 2 2 5
Fig. 1 e XRD pattern of the Pt/ND catalyst.
authors coated platinum (Pt) nanoparticles on undoped ND powder with average particle sizes of 5 and 100 nm using electrodeposition from 1.1 mmol/L chloroplatinic acid. The Pt/ ND nanoparticles exhibited good electrocatalytic properties for methanol electrooxidation [26]. The microwave-assisted polyol process, a rapid and uniform heating method, has received considerable attention
1221
as a promising method to synthesize nanosized materials [27e32]. In this method, ethylene glycol (EG) is commonly used as a solvent and reducing agent for metal salt compounds as well as a protecting agent for metal nanoparticles. The polymer-stabilized Pt, Ru, Ag, and Pd colloids with small and uniform particle sizes have been prepared using the microwave-assisted polyol process [30,33e36]. In previous studies, Pt/XC-72 carbon [37], Pt/graphite nanofibers [38] and Pt/CNTs [39] were prepared using this process and showed very good electrocatalytic properties for methanol electrooxidation. In the present work, Pt was coated on the surface of ND powder by microwave-heating EG solutions of platinum precursor salt. The electrocatalytic effects of the catalysts with different Pt mass percentages on methanol oxidation were investigated.
2.
Experimental
ND powder with an average particle size of 50 nm was fabricated by mechanical crushing and was supplied by Henan Boreas Technological Development Co. A platinum precursor (H2PtCl6$6H2O), EG, and sulfuric acid (H2SO4) were purchased from Shanghai Chemical Products Ltd. Deionized water was
Fig. 2 e TEM images of (a) Pt/ND-a, (b) Pt/ND-b, (c) Pt/ND-c, and (d) Pt/ND-d.
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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 7 ( 2 0 1 2 ) 1 2 2 0 e1 2 2 5
used to prepare the solutions, and high-purity nitrogen gas was also used in the experiments. In a typical procedure, 2.0 mL aqueous H2PtCl6 solution (0.055 mol/L) was mixed with 25 mL EG in a 100 mL beaker. Then, the mixture was uniformly mixed with 80, 40, 20, or 10 mg of 50 nm ND using ultrasound to obtain different Pt mass percentages. The beaker was placed at the center of a microwave oven and was heated for 80 s at 800 W. The resulting suspension was filtered, and the residue was washed with acetone and deionized water. The solid products were dried at 100 C for 12 h in a vacuum oven. Pt/ND-a, Pt/ND-b, Pt/ ND-c, and Pt/ND-d samples, corresponding to the addition of 80, 40, 20, and 10 mg ND in the mixtures were obtained, respectively. Energy dispersion spectra (EDS) analysis was performed to determine the metal mass fraction of the Pt/ND catalysts. Xray diffraction (XRD) was employed to determine the crystalline structure of the samples. A JEM-2010 high-resolution transmission electron microscope (TEM) was used to characterize the morphology of the catalysts. Electrocatalytic activities of Pt/ND electrocatalysts for methanol electrooxidation were measured by cyclic voltammetry (CV) and chronoamperometry (CA) using a CHI660A electrochemical workstation. A conventional three-electrode system was used, consisting of a Pt/ND electrode as the working electrode, a platinum coil auxiliary electrode, and an Ag/AgCl electrode as the reference electrode. The experiments were conducted at 25 C. The working electrode was obtained as follows: Pt/ND catalysts (10 mg) were dispersed in 10 mL ethanol by ultrasonication for 30 min to achieve a 1.0 mg/mL suspension, and one drop of the suspension was directly cast on the surface of the glassy carbon electrode and evaporated at room temperature. The active specific surface area of the Pt particles was calculated according to a hydrogen electrosorption curve. The potential was cycled between þ1.0 and 0.3 V in 1.0 mol/L H2SO4 electrolyte at 50 mV/s to obtain the voltammograms of hydrogen adsorption. All electrolyte solutions were deaerated by high-purity nitrogen for 30 min prior to any electrochemical measurement.
3.
Results and discussion
3.1.
XRD characterizations of Pt/ND catalysts
d (d) catalysts prepared using the microwave polyol process with different Pt mass percentages. Table 1 lists the corresponding relations between the ratio of Pt to ND support in the starting mixture and the Pt mass percentage in Pt/ND electrocatalysts based on the EDS analysis. H2PtCl6 added in the starting mixture was suggested to be almost totally reduced. Fig. 2 shows that the Pt nanoparticles are dispersed on the facet surfaces of ND, and that these nanoparticles are approximately 4e5 nm and are uniformly distributed. Pt nanoparticles become denser on the surface of ND as the Pt mass percentage of the Pt/ND nanocomposite increases. When the Pt mass percentage is 50.1%e51.5% (Pt/ND-c), the ND surface is almost studded with Pt particles (Fig. 2c). Further increase in Pt mass percentage results in the onset of agglomeration of Pt particles (Fig. 2d), which could lead to a reduction on the surface area of Pt.
3.3.
Methanol electrooxidization over Pt/ND catalysts
Fig. 4 shows the CV curves of methanol electrooxidization by the prepared catalysts in 1.0 mol/L CH3OHþ0.5 mol/L H2SO4 solutions. Two peaks of methanol oxidation are observed at 0.64 and 0.43 V. The characteristics of the CV curves and peak potentials (Ep) are similar to those of other catalysts with sp2bonded carbon as support [40,41], proving the catalytic activity of the Pt/ND composite toward methanol oxidation. As shown in Fig. 4, Pt/ND-a displays the lowest peak current (curve a) for methanol electrooxidation among the Pt/ND catalysts. The
The structure of the Pt/ND composite was characterized by XRD (Fig. 1). The XRD measurement confirmed the presence of crystalline Pt nanoparticles with characteristic peaks at 2q values of 39.8 , 46.2 , 67.4 , and 81.2 , corresponding to the reflection planes (1 1 1), (2 0 0), (2 2 0), and (3 1 1), respectively. The peaks could be indexed to face-centered cubic platinum, and the widening of the peaks implies the nanoscale characteristic of the Pt particles. The most intense peaks appeared at 43.98 , 75.28 , and 91.68 , corresponding to the (1 1 1), (2 2 0), and (3 1 1) cubic diamond planes.
3.2.
TEM and EDS characterizations of Pt/ND catalysts
Figs. 2 and 3 show the TEM images and the corresponding EDS spectra of the Pt/ND-a (a), Pt/ND-b (b), Pt/ND-c (c), and Pt/ND-
Fig. 3 e EDS spectra of (a) Pt/ND-a, (b) Pt/ND-b, (c) Pt/ND-c, and (d) Pt/ND-d.
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Table 1 e Comparison of calculated and measured values of Pt mass percentage. Samples
Pt/ND-a Pt/ND-b Pt/ND-c Pt/ND-d
Calculated ND H2PtCl6 Measured values mass (55 mmol/L) values of (mL) of Pt mass (mg) Pt mass percentage percentage (%) (%) (from EDS) 80 40 20 10
2 2 2 2
21.2 34.9 51.8 68.2
20.6e21.6 31.5e35.4 50.1e51.5 65.6e66.9
low electrocatalytic activity is due to the sparse distribution of the Pt particle on the ND surface (Fig. 2a). When the Pt mass percentage is increased, Pt/ND catalysts display higher peak currents for methanol electrooxidation. The highest oxidation current occurs on Pt/ND-c (curve c), whose surface presents the most uniform and dense Pt nanoparticles (Fig. 2c). However, with increasing Pt mass percentage (Pt/ND-d), peak currents begin to decrease significantly (curve d) because of the agglomeration of Pt particles on the surface (Fig. 2d). The CA profiles of methanol electrooxidation on Pt/ND electrodes at 0.6 V are displayed in Fig. 5. The Pt/ND-c catalysts (curve c) exhibit not only high initial current, but also a higher current compared with those of the other prepared catalysts (curve a, b, and d). This characteristic indicates that catalysts with 50.1%e51.5% Pt mass percentages yield better performance. The currents of methanol electrooxidation on the Pt/C catalysts decrease moderately with time (Fig. 5). The current decay is due to the intermediate products of methanol oxidization such as CO and other ions in the electrolyte adsorbed onto the Pt nanoparticles that inhibit methanol electrooxidization. The Pt/ND-d catalysts (curve d in Fig. 5) exhibited not only lower currents compared with Pt/ND-c (curve c in Fig. 5), but also a faster decrease in speed compared with that of Pt/ND-b (curve b in Fig. 5). These results indicate that the agglomeration of Pt particles on the ND surface leads to an unstable electrocatalytic activity.
Fig. 4 e CV curves of methanol electrooxidization on (a) Pt/ ND-a, (b) Pt/ND-b, (c) Pt/ND-c, (d) Pt/ND-d in 1.0 mol/L CH3OHD0.5 mol/L H2SO4.
Fig. 5 e CA curves of (a) Pt/ND100-a, (b) Pt/ND100-b, (c) Pt/ ND100-c, and (d) Pt/ND100-d electrodes in 1.0 mol/L CH3OHD0.5 mol/L H2SO4.
3.4.
Active specific surface area of Pt/ND catalysts
The actual surface area of Pt particles is generally one of the important parameters in determining the catalytic properties of catalysts for methanol electrooxidation considering that this reaction is surface sensitive. The active specific surface area of Pt particles for Pt/ND-c catalysts can be estimated from the integrated charge in the hydrogen adsorption region of the CV curves (hatched area of Fig. 6). The areas (m2/g) were calculated from the following formula, assuming a correspondence value of 0.21 mC/cm2, a value generally used for polycrystalline Pt electrodes [42], and Pt loading: AEL m2 =g Pt ¼ QH = 0:21 103 C ðg ptÞ
(1)
where AEL is the active specific surface area of Pt particles obtained electrochemically, QH is the amount of charge exchanged during the electro-adsorption of hydrogen atoms
Fig. 6 e Hydrogen electro-adsorption CV curve for Pt/ ND100-c in 1.0 mol/L H2SO4, v [ 0.05 V/s.
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on Pt, and C is the unit Coulomb. The obtained AEL of the Pt/ ND-c catalysts was 64.5 m2/g Pt, corresponding approximately to that of Pt/C catalysts with Pt nanoparticles of the same size [43].
4.
Conclusions
Pt/ND catalysts with different Pt mass percentages were synthesized using a microwave-assisted polyol method. The mean size of the Pt particles was 4e5 nm. Pt/ND catalysts with 50.1%e51.5% Pt mass percentages exhibited better electrocatalytic performance for methanol oxidation. The improvement in electrocatalytic performance was because the fact that the prepared Pt/ND catalyst had small, uniform, dense, and highly dispersed Pt nanoparticles when a proper amount of chloroplatinic acid was added to the synthesis solution. However, further increase in Pt mass percentage resulted in the agglomeration Pt particles on the ND surface, leading to a worse catalytic performance. The microwave-assisted polyol method for preparing Pt/ ND catalysts in the present paper is simple, effective, and can be used as an alternative process for preparing highperformance electrocatalysts.
Acknowledgements This work was funded by the National Natural Science Foundation of China (Grant Nos. 50872119 and 50972125) and the Natural Science Foundation of Hebei Province (Grant Nos. E2011203126 and E2010001187). The authors also gratefully acknowledge the funding from Element Six Co. J.B. Zang thank the China Postdoctoral Science Foundation (Grant No. 20100480197) as well for its assistance.
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