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Synthesis and electrocatalytic performance of MWCNT-supported Ag@Pt core–shell nanoparticles for ORR
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 3 3 6 5 e1 3 3 7 0
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Synthesis and electrocatalytic performance of MWCNT-supported Ag@Pt coreeshell nanoparticles for ORR Shuping Yu, Qun Lou, Kefei Han, Zhongming Wang, Hong Zhu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
article info
abstract
Article history:
Ag@Pt coreeshell nanoparticles with different Ag/Pt ratios were supported on multi walled
Received 30 May 2012
carbon nanotubes (MWCNTs) and used as electrocatalysts for PEMFC. The morphology of
Received in revised form
the electrocatalyst samples was characterized by XRD and HRTEM. It was found that the
27 June 2012
Ag@Pt/MWCNTs catalyst exhibited a coreeshell nanostructure. And the CV and LSV results
Accepted 28 June 2012
demonstrated that such coreeshell materials exhibited attractive electrocatalytic activity.
Available online 25 July 2012
Moreover, the specific electrochemically active area (EAS) of the Ag@Pt/MWCNTs catalyst is 70.63 m2 g1, which is higher than the values reported in the literature.
Keywords:
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Coreeshell
reserved.
Low Pt loading Electrocatalyst Oxygen reduction PEMFC
1.
Introduction
Electrocatalysts play an important role in proton exchange membrane fuel cells (PEMFCs) [1,2]. Electrocatalysts can solve the problem of high electrochemical polarization of oxygen reduction, which leads to great loss of battery efficiency and output voltage [3,4]. Pt electrocatalyst is regarded as the most important electrocatalyst materials because of its high activity [5,6], however, Pt is a rare and precious metal. Thus reducing Pt usageand improving the catalytic activity have become the critical issues in fuel cells [7e10]. It is well-known that the catalytic activity is strongly dependent on the shape, size, distribution and structure of the catalyst nanoparticles. The inter-metallic Pt electrocatalysts with cheap transition metal cores exhibiting enhanced electrocatalytic performance for PEMFC have been widely used. For example, Oh et al. [11] used g-irradiationinduced reduction of metal ions to produce dispersions of Ag,
Pd, and PteRu alloy nanoparticles in poly vinylpyrrodione (PVP) at room temperature. They developed a novel method to synthesize alloy catalysts. Nadagouda and Varma [12] used a green method to fabricate novel core (Fe and Cu)-shell (noble metals) metal nanocrystals by using aqueous ascorbic acid (vitamin C). These nanocrystals have unique properties that have not originally presented in either the core or shell materials and may have potential functions in catalysis and other technological applications. A coreeshell structured Ru@PtPd/C catalyst synthesized by Gao et al. [13] showed high activity towards the anodic oxidation of ethanol. Its activity in terms of PtPd loading is 1.3, 3, 1.4, and 2.0 times as high as that of PtPd/C, PtRu/C, Pd/C, and Pt/C, respectively, indicating high utilization of Pt and Pd. Recently, R. R. Adzica and coworkers [14] established a scale-up synthesis method to produce gram-quantities of Pt monolayer on Pd nanoparticle surfaces. The Pt mass activity of the Pt/Pd/C electrocatalyst for oxygen reduction reaction (ORR) is
* Corresponding author. Tel.: þ86 10 64411443; fax: þ86 10 64444919. E-mail address: [email protected] (H. Zhu). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.06.109
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considerably higher than that of commercial Pt/C electrocatalysts. Recent studies show that coreeshell catalysts possess very high ORR activity for PEMFCs. For example, Mani et al. [15] synthesized a novel class of PteCu coreeshell nanoparticle materials for using as oxygen reduction electrocatalyst in PEMFC. The mass activity of the electrocatalytic Pt for ORR exceeds that of state-of-the art Pt electrocatalyst by more than a factor of 4 and thus are able to meet performance targets for fuel cell cathodes. These authors proposed a hypothesis that a reduced PtePt distance of lattice unit near the particle surface, stabilized by the lattice-contracted alloy core, may explain the modification of the surface catalytic reactivity. Yang et al. [16] reported on the use of carbon-supported Ni core-Pt shell Ni1-x@Ptx/C (x ¼ 0.32, 0.43, 0.60, 0.67, and 0.80) nanoparticles as catalysts. The diameter of the Ni core-Pt shell nanoparticles was 2e4 nm. The facile synthesis of trimetallic nanoparticles with PdCu alloy as cores and Pt as shells was reported by Wang et al. [17]. Pseudo-core-shell catalyst with incomplete and thin shell of Pt on PdCu has a higher catalytic activity than Pt/C and PtRu/C as a result of improved Pt utilization. Zhu et al. [18] synthesized Cu@Pt/C catalyst by a twostep reduction method which useVulcan XC-72R as the supporting material, and the ECSA (Electrochemical active surface area) of Pt is 53.81 m2 g1. Consequently, many researchers have been searching for ways to minimize or eliminate the usage of Pt-like precious metals in catalysts. An effective way is to make alloys with a second or third metal or a coreeshell structure. It is a convenient way to modify the electrocatalytic properties of Pt in order to reduce Pt usage [19e21]. The formation of alloys of transitionmetals can change the outer electronarrangement of Pt atom, and this kind of alloys benefits the double adsorption and dissociation of oxygen on the surface Pt [22]. Ag is a transition metal with face-centered cubic crystals. It is expected to be applied in the electrocatalysts for PEMFC. The lattice spacing of Ag is closer to that of Pt. The similarity in lattice spacing facilitates the growth of Pt on the Ag core [23,24]. Multi-walled carbon nanotubes with their large surface area and good electrical conductivity can be a good support for electrocatalyst [25]. In this study, Ag@Pt coreeshell nanoparticles on multiwalled carbon nanotubes (MWCNTs) support and with appropriate Pt to Ag mass ratio was synthesized by using NaBH4 and ethylene glycol as reducing agent and solvent, respectively. Silver sol was loaded on the carbon nanotubes, and then loading Pt nanoparticles on the surface of silver
nanoparticles. The formation of coreeshell structure was confirmed by HRTEM, XRD, and electrochemical techniques. Improved Pt utilization and the best mass ratio of Pt to Ag were studied by experiments.
2.
Experimental
2.1.
Preparation of Ag@Pt/MWCNTs electrocatalyst
An Ag@Pt/MWCNTs electrocatalyst was synthesized by a colloidal method and a chemical reduction method with NaBH4 and ethylene glycol as reducing agents. In a typical process, an Ag@Pt/MWCNTs electrocatalyst with a nominal mass ratio Ag/Pt of 1/1 (the quantity of metal accounts for 40% of the total mass of the electrocatalyst) was prepared as follows. First, MWCNTs were pretreated in boiling 16 M HNO3 for 4 h. Second, citrate-stabilized Ag seeds were prepared from the NaBH4 reduction of AgNO3. Then a predetermined amount of the MWCNTs and 370 mL of 1 mM aqueous AgNO3 solution were mixed with 1.6 mL of 38.8 mM aqueous sodium citrate solution (used as a stabilizer). A 37 mL sample of 22.4 mM aqueous NaBH4 solution was added dropwise under vigorous stirring, giving rise to a yellow Ag hydrosol. The Ag hydrosol was aged for 24 h to decompose the residual NaBH4 before it was used in subsequent steps. Third, the obtained Ag/ MWCNTs powder was dispersed in ethylene glycol (EG). Appropriate amounts of H2PtCl6$6H2O aqueous solution were added to a fiask. The pH of the system was adjusted to 7e8 with KOH/EG solution dropped. After the mixture was stirred for 4 h at 90 C, the Ag@Pt/MWCNTs electrocatalyst was collected. Ag@Pt/MWCNTs electrocatalysts with different mass ratios of Ag to Pt were synthesized according to the procedure described above by keeping the total metal loading at 40 wt%. For comparison, a Pt/MWCNTs catalyst with the same Pt content as Ag@Pt/MWCNTs was synthesized by using the method described above. Scheme 1 depicts the synthesis procedure for the Ag/MWCNTs nanomaterials and the subsequent deposition of a Pt layer. In this process, the oxygen functional groups (COOHe, OHe, COHe) on the surface of MWCNTs become active sites, which can absorb Agþ. Therefore, Ag nanoparticles can form easily and act as the core, and the Pt covering the Ag nanoparticles acts as the shell. The formation of coreeshell structure depends upon the metal salt solution and the reducing/stabilizing agent used during the preparation.
Scheme 1 e Schematic diagram for the formation of Ag@Pt/MWCNTs coreeshell nanostructure.
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2.2.
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Characterization
The morphology of the electrocatalyst was determined by high resolution transmission electron microscopy (HRTEM) on a JEOL S-520 microscope. X-ray diffraction (XRD) analysis was performed on a D/max-2200/PC X-ray diffractometer with a Cu Ka radiation source. The particle size was calculated by using the Scherrer equation [26]: D ¼ 0:9l=Bcos q
(1)
˚ , l is the X-ray wavewhere D is the diameter of particle in A length, q is the Bragg angle, and B is the full width at half maximum (FWHM, in radians) of the peak under consideration. Electrochemical measurements were carried out by using a Zahner Ennium electrochemical workstation with a threeelectrode cell. For cyclic voltammetry (CV) measurements and linear sweep voltammetry (LSV) measurements, a glassy carbon disk coated with electrocatalyst was used as the working electrode. The catalyst ink was produced by mixing 5 mg of the catalyst under sonication for 2 h in 0.1 ml Nafion solution and 0.9 ml ethanol. 5 ml or 10 ml of the electrocatalyst ink was dropped on the clean glassy carbon disk (3 mm or 5 mm outer diameter) and dried at room temperature. A Pt needle and a standard calomel electrode (SCE) were used as the counter-electrode and the reference electrode, respectively. The CV tests were conducted at 50 mV s1 in the potential range 0.2e1.2 V (vs. SCE). The LSV tests were conducted at 2 mV s1 in the potential range 0e0.8 V (vs. SCE).
3.
Results and discussion
Evidence for an AgePt coreeshell in Ag@Pt/MWCNTs can be seen in the XRD patterns in Fig. 1 (a). For comparison purposes, the XRD patterns for Ag/MWCNTs and Pt/MWCNTs prepared by a similar procedure are also shown in Fig. 1 (a). The peak located at about 2q ¼ 26.0 in all the XRD patterns is associated with the multi-walled carbon nanotube as supporter. The diffractions of Ag/MWCNTs (curve a in Fig. 1 (a)) matched well with those of the Ag characteristic peaks (2q ¼ 38.1, 44.2, and 64.4 ) (PDF card 87e0597). In curve b, the four peaks at 2q values of 39.8 , 46.2 , 67.4 and 81.3 are characteristics of the face-centered cubic (fcc) crystalline alloys of Pt (PDF card 04e0802), corresponding to the planes (111), (200), (220), and (311), respectively. In contrast, the 2q angles of characteristic diffractions peaks of Ag@Pt/MWCNTs as shown in curve c in Fig. 1 (a) were significantly lower than those of Pt/MWCNTs. The crystallinity of Ag is not obvious in Ag@Pt/MWCNTs, an indication that the Ag surface in Ag@Pt is mostly wrapped by Pt. This result is expected because the lattice constant for fcc Ag is larger than that for Pt [27]. According to the Bragg equation, the small negative shift of the Pt crystal characteristic peaks implies that the Pt lattice has expanded to cover the Ag surface, forming an Ag@Pt structure. The mean particle size of Ag@Pt nanoparticles was estimated by the Scherrer formula to be 14.2 nm in the Pt (111) lattice. The XRD results of Ag@Pt/MWCNTs catalysts with different mass ratios of Ag to Pt shown in Fig. 1 (b)
Fig. 1 e (a) XRD patterns of Ag/MWCNTs, Pt/MWCNTs, and Ag@Pt/MWCNTs catalysts. (b) XRD patterns of Ag@Pt/ MWCNTs catalysts with different Ag/Pt mass ratios at 40 wt% total metal. Insert figure: enlargement for characteristic diffraction peaks of Pt (111) and Pt (220) crystal surface.
demonstrate that with the decrease of Pt and increase of Ag, the extent of negative shift in the peak for Pt in Ag@Pt/ MWCNTs increases gradually. At the same time, the characteristic peaks for Pt becomes weaker, which confirms that the Pt lattice has contracted by loading on the Ag particles [28]. The insets in Fig. 1 (b) show the enlargement of the characteristic peaks of the plane (111) and (220), clearly indicating the negative shifts. Fig. 2 (a) shows that the catalysts after the loading of Pt are highly dispersed on the MWCNTs surface. Different from the faceted cubic or octahedral shape, the multi-walled carbon nanotubes supported metal (alloy) nanoparticles exhibited a spherical or elliptical shape. The coreeshell Ag@Pt particles with diameters from 12 to 17 nm were uniformly dispersed on the MWCNTs support. The lattice spacing of the Ag crystalline (111) plane is 0.236 nm, while the lattice spacing of the Pt crystalline (111) plane is 0.228 nm. As is shown in Fig. 2 (b), the coreeshell structure of nanoparticle is observed.
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Fig. 2 e (a) TEM image of Ag@Pt/MWCNTs catalyst. (b) HRTEM image of Ag@Pt catalyst.
Simultaneously, the lattice spacing of the PtAg crystalline (111) plane is 0.232 nm, which is between the lattice spacings of Pt and Ag, implying that the lattice spacing of Pt has expanded [18], in agreement with the XRD results shown in Fig. 1 There are two reasons for the formation of the coreeshell structure: first, the interaction between the metals is stronger than that between either metal and the carbon; second, both Pt crystallites and Ag crystallites have a facecentered cubic structure, a phenomenon in favor of the growth of Pt nanoparticles on the Ag surface [29]. The electrocatalytic activity of the Ag@Pt/MWCNTs catalysts was characterized by CV, and the results are shown in Fig. 3. Fig. 3 (a) shows the cyclic voltammograms of Ag@Pt/ MWCNTs and Pt/MWCNTs at same Pt loading. The catalysts exhibit H2 adsorption/desorption peaks in the potential range 0.2 to 0 V (vs. SCE). The Pt/MWCNTs scan exhibits two distinct potential regions, a typical voltammetric response of Pt surface under acidic conditions. In the hydrogen adsorption/desorption region, pure Pt shows two resolved peaks associated with weakly and strongly bonded hydrogen species on different crystal faces of Pt [30]. Furthermore, Ag@Pt/ MWCNTs exhibites well-defined current peaks associated with hydrogen adsorption-desorption processes. The cyclic voltammograms of Ag@Pt/MWCNTs coreeshell electrocatalysts with different mass ratios of Ag to Pt are shown in Fig. 3 (b). The electrochemical active surface area (EAS) of the catalysts was calculated according to Eq. (2) [31]: EAS m2 $g1 ¼ QH =ð2:1 ½PtÞ
(2)
2
where QH (C m ) is the charge exchanged during hydrogen desorption on the Pt surface and [Pt] (g$m2) is the Pt loading on the electrode. The calculated electrochemical active surface areas (EAS) of the catalysts are given in Table 1. According to Table 1, the EAS and MSA (Mass specific activity) of the H2 adsorption/desorption peaks are larger for Ag@Pt/ MWCNTs6 catalysts than for Pt/MWCNTs1 catalysts, indicating that the catalytic activity and utilization of Pt are better for coreeshell nanostructure catalysts than for pure Pt catalysts. The EAS of the Ag@Pt/MWCNTs4 catalyst with an Ag to Pt mass ratio of 10 wt% to 30 wt% was calculated to be
Fig. 3 e (a) Cyclic voltammograms of Ag@Pt/MWCNTs coreeshell electrocatalyst and Pt/MWCNTs catalyst with same Pt loading. (b) Cyclic voltammograms of Ag@Pt/ MWCNTs electrocatalysts with different Ag: Pt mass ratios. The electrolyte was 0.5 M H2SO4. Scanning rate was 50 mV sL1.
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Table 1 e Atomic Pt/Ag ratio, EAS, and MSA of Ag@Pt/MWCNTs electrocatalyst. Catalysts
Metal loading (wt %)
1
Pt/MWCNTs Pt/MWCNTs2 Ag@Pt/MWCNTs3 Ag@Pt/MWCNTs4 Ag@Pt/MWCNTs5 Ag@Pt/MWCNTs6
Pt
Ag
20 40 35 30 25 20
e e 5 10 15 20
Atomic Pt/Ag ratio
EAS (m2/g-Pt)
MSAa (mA/mg-Pt)
e e 3.89 1.66 0.92 0.55
31.18 40.96 55.59 70.63 34.73 33.54
42.0 49.9 63.9 56.3 60.0 51.8
a Mass specific activity of platinum at the peak potential (0.47 V vs. SCE) of oxygen reduction.
nanoparticles was about 14.2 nm, and the nanoparticles were well dispersed on the surface of MWCNT. The EAS of the Ag@Pt/MWCNTs catalyst was 70.63 m2 g1 at an Ag: Pt mass ratio of 10:30, and the atomic ratio Pt/Ag was 1.66. The mass content of Pt in the Ag@Pt/MWCNTs catalyst was lower than the value of 40% in the commercial Johnson-Matthey (JM) electrocatalyst. All the results show that the Ag@Pt/MWCNTs catalyst exhibits much higher catalytic activity than pure Pt catalyst and is very promising for PEMFC applications.
Acknowledgments
Fig. 4 e Polarization curves of Ag@Pt/MWCNTs and Pt/ MWCNTs catalysts in O2-saturated 0.5 mol LL1 H2SO4 at a rotation rate of 1600 rpm. Sweep rate: 2 mV sL1.
70.63 m2 g1. The MSA of all coreeshell electrocatalysts are large than that of pure Pt electrocatalysts. Fig. 4 shows that the onset reduction potential shifts 40 mV towards positive potential for the Ag@Pt/MWCNTs catalyst with respect to the corresponding pure Pt/MWCNTs catalyst. On the one hand, the surface oxide formation (Pt þ H2O / PteOH þ Hþ þ e) and the subsequent reduction of the surface oxide occur in the potential range [30]. This result shows that the oxygenated Pt species (PteOH species) have a weaker adsorption energy on the surface of the Ag@Pt/ MWCNTs catalyst than on the surface of the pure Pt catalyst, which means that the desorption of oxygenated Pt species is easier on Ag@Pt/MWCNTs than on the Pt/MWCNT [32]. The easier desorption on Ag@Pt/MWCNTs should facilitate the ORR and four-electron reduction reaction. On the other hand, the electrocatalytic activity enhancement for oxygen reduction is the result of the larger surface for Ag@Pt/MWCNTs, which provides more active sites.
4.
Conclusion
An Ag@Pt/MWCNTs electrocatalyst was synthesized by using NaBH4 and ethylene glycol as reducing agent and solvent, respectively. The average particle size of the Ag@Pt
The authors gratefully acknowledge the financial supports from the National Science Foundation of China (21176022), The International ration Program of China (No. 2009DFA63120), National High Technology Research and Development Program of China (No. 2011AA11A273 and No. 2012AA03A709) and National Defense Basic Scientific Research Program of China (No: A1420110023)
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[8] Wei YC, Liu CW, Lee HW, Chung SR, Lee SL, Chan TS, et al. Synergistic effect of Co alloying and surface oxidation on oxygen reduction reaction performance for the Pd electrocatalysts. Int J Hydrogen Energy 2011;36: 3789e802. [9] Zhao J, Sarkar A, Manthiram A. Synthesis and characterization of Pd-Ni nanoalloy electrocatalysts for oxygen reduction reaction in fuel cells. Electrochimi Acta 2010;55:1756e65. [10] Liao MJ, Wei ZD, Chen SG, Li L, Ji MB, Wang YQ. Ultra low Ptloading electrode prepared by displacement of electrodeposited Cu particles on a porous carbon electrode. Int J Hydrogen Energy 2012;35:8071e9. [11] Oh SD, So BK, Choi SH, Gopalan A, Lee KP, Yoon KR, et al. Dispersing of Ag, Pd, and Pt-Ru alloy nanoparticles on singlewalled carbon nanotubes by g-irradiation. Mater Lett 2005;59: 1121e4. [12] Nadagouda MN, Varma RS. A Greener synthesis of core (Fe, Cu)-shell (Au, Pt, Pd, and Ag) nanocrystals using aqueous vitamin C. Cryst Growth Des 2007;7:2582e7. [13] Gao HL, Liao SJ, Liang ZX, Liang HG, Luo F. Anodic oxidation of ethanol on core-shell structured Ru@PtPd/C catalyst in alkaline media. J Power Sources 2011;196:6138e43. [14] Sasakia K, Wang JX, Naohara H, Marinkovic N, More K, Inada H, et al. Recent advances in platinum monolayer electrocatalysts for oxygen reduction reaction: scale-up synthesis, structure and activity of Pt shells on Pd cores. Electrochimi Acta 2010;55:2645e52. [15] Mani P, Srivastava R, Strasser P. Dealloyed Pt-Cu core-shell nanoparticle electrocatalysts for use in PEM fuel cell Cathodes. J Phys Chem C 2008;112:2770e8. [16] Yang XJ, Cheng FY, Liang J, Tao ZL, Chen J. Carbon-supported Ni1-x@Ptx (x¼0.32, 0.43, 0.60, 0.67, and 0.80) core-shell nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane. Int J Hydrogen Energy 2011; 36:1984e90. [17] Wang H, Wang RF, Li H, Wang QF, Kang J, Lei ZQ. Facile synthesis of carbon-supported pseudo-core@shell PdCu@Pt nanoparticles for direct methanol fuel cells. Int J Hydrogen Energy 2011;36:839e48. [18] Zhu H, Li XW, Wang FH. Facile synthesis of carbonsupported pseudo-core@shell PdCu@Pt nanoparticles for direct methanol fuel cells. Int J Hydrogen Energy 2011;36: 9151e4. [19] Wang RF, Wang H, Wei BX, Wang W, Lei ZQ. Carbon supported Pt-shell modified PdCo-core with electrocatalyst for methanol oxidation. Int J Hydrogen Energy 2010;35: 10081e6.
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Synthesis and electrocatalytic performance of MWCNT-supported Ag@Pt coreeshell nanoparticles for ORR Shuping Yu, Qun Lou, Kefei Han, Zhongming Wang, Hong Zhu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
article info
abstract
Article history:
Ag@Pt coreeshell nanoparticles with different Ag/Pt ratios were supported on multi walled
Received 30 May 2012
carbon nanotubes (MWCNTs) and used as electrocatalysts for PEMFC. The morphology of
Received in revised form
the electrocatalyst samples was characterized by XRD and HRTEM. It was found that the
27 June 2012
Ag@Pt/MWCNTs catalyst exhibited a coreeshell nanostructure. And the CV and LSV results
Accepted 28 June 2012
demonstrated that such coreeshell materials exhibited attractive electrocatalytic activity.
Available online 25 July 2012
Moreover, the specific electrochemically active area (EAS) of the Ag@Pt/MWCNTs catalyst is 70.63 m2 g1, which is higher than the values reported in the literature.
Keywords:
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Coreeshell
reserved.
Low Pt loading Electrocatalyst Oxygen reduction PEMFC
1.
Introduction
Electrocatalysts play an important role in proton exchange membrane fuel cells (PEMFCs) [1,2]. Electrocatalysts can solve the problem of high electrochemical polarization of oxygen reduction, which leads to great loss of battery efficiency and output voltage [3,4]. Pt electrocatalyst is regarded as the most important electrocatalyst materials because of its high activity [5,6], however, Pt is a rare and precious metal. Thus reducing Pt usageand improving the catalytic activity have become the critical issues in fuel cells [7e10]. It is well-known that the catalytic activity is strongly dependent on the shape, size, distribution and structure of the catalyst nanoparticles. The inter-metallic Pt electrocatalysts with cheap transition metal cores exhibiting enhanced electrocatalytic performance for PEMFC have been widely used. For example, Oh et al. [11] used g-irradiationinduced reduction of metal ions to produce dispersions of Ag,
Pd, and PteRu alloy nanoparticles in poly vinylpyrrodione (PVP) at room temperature. They developed a novel method to synthesize alloy catalysts. Nadagouda and Varma [12] used a green method to fabricate novel core (Fe and Cu)-shell (noble metals) metal nanocrystals by using aqueous ascorbic acid (vitamin C). These nanocrystals have unique properties that have not originally presented in either the core or shell materials and may have potential functions in catalysis and other technological applications. A coreeshell structured Ru@PtPd/C catalyst synthesized by Gao et al. [13] showed high activity towards the anodic oxidation of ethanol. Its activity in terms of PtPd loading is 1.3, 3, 1.4, and 2.0 times as high as that of PtPd/C, PtRu/C, Pd/C, and Pt/C, respectively, indicating high utilization of Pt and Pd. Recently, R. R. Adzica and coworkers [14] established a scale-up synthesis method to produce gram-quantities of Pt monolayer on Pd nanoparticle surfaces. The Pt mass activity of the Pt/Pd/C electrocatalyst for oxygen reduction reaction (ORR) is
* Corresponding author. Tel.: þ86 10 64411443; fax: þ86 10 64444919. E-mail address: [email protected] (H. Zhu). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.06.109
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considerably higher than that of commercial Pt/C electrocatalysts. Recent studies show that coreeshell catalysts possess very high ORR activity for PEMFCs. For example, Mani et al. [15] synthesized a novel class of PteCu coreeshell nanoparticle materials for using as oxygen reduction electrocatalyst in PEMFC. The mass activity of the electrocatalytic Pt for ORR exceeds that of state-of-the art Pt electrocatalyst by more than a factor of 4 and thus are able to meet performance targets for fuel cell cathodes. These authors proposed a hypothesis that a reduced PtePt distance of lattice unit near the particle surface, stabilized by the lattice-contracted alloy core, may explain the modification of the surface catalytic reactivity. Yang et al. [16] reported on the use of carbon-supported Ni core-Pt shell Ni1-x@Ptx/C (x ¼ 0.32, 0.43, 0.60, 0.67, and 0.80) nanoparticles as catalysts. The diameter of the Ni core-Pt shell nanoparticles was 2e4 nm. The facile synthesis of trimetallic nanoparticles with PdCu alloy as cores and Pt as shells was reported by Wang et al. [17]. Pseudo-core-shell catalyst with incomplete and thin shell of Pt on PdCu has a higher catalytic activity than Pt/C and PtRu/C as a result of improved Pt utilization. Zhu et al. [18] synthesized Cu@Pt/C catalyst by a twostep reduction method which useVulcan XC-72R as the supporting material, and the ECSA (Electrochemical active surface area) of Pt is 53.81 m2 g1. Consequently, many researchers have been searching for ways to minimize or eliminate the usage of Pt-like precious metals in catalysts. An effective way is to make alloys with a second or third metal or a coreeshell structure. It is a convenient way to modify the electrocatalytic properties of Pt in order to reduce Pt usage [19e21]. The formation of alloys of transitionmetals can change the outer electronarrangement of Pt atom, and this kind of alloys benefits the double adsorption and dissociation of oxygen on the surface Pt [22]. Ag is a transition metal with face-centered cubic crystals. It is expected to be applied in the electrocatalysts for PEMFC. The lattice spacing of Ag is closer to that of Pt. The similarity in lattice spacing facilitates the growth of Pt on the Ag core [23,24]. Multi-walled carbon nanotubes with their large surface area and good electrical conductivity can be a good support for electrocatalyst [25]. In this study, Ag@Pt coreeshell nanoparticles on multiwalled carbon nanotubes (MWCNTs) support and with appropriate Pt to Ag mass ratio was synthesized by using NaBH4 and ethylene glycol as reducing agent and solvent, respectively. Silver sol was loaded on the carbon nanotubes, and then loading Pt nanoparticles on the surface of silver
nanoparticles. The formation of coreeshell structure was confirmed by HRTEM, XRD, and electrochemical techniques. Improved Pt utilization and the best mass ratio of Pt to Ag were studied by experiments.
2.
Experimental
2.1.
Preparation of Ag@Pt/MWCNTs electrocatalyst
An Ag@Pt/MWCNTs electrocatalyst was synthesized by a colloidal method and a chemical reduction method with NaBH4 and ethylene glycol as reducing agents. In a typical process, an Ag@Pt/MWCNTs electrocatalyst with a nominal mass ratio Ag/Pt of 1/1 (the quantity of metal accounts for 40% of the total mass of the electrocatalyst) was prepared as follows. First, MWCNTs were pretreated in boiling 16 M HNO3 for 4 h. Second, citrate-stabilized Ag seeds were prepared from the NaBH4 reduction of AgNO3. Then a predetermined amount of the MWCNTs and 370 mL of 1 mM aqueous AgNO3 solution were mixed with 1.6 mL of 38.8 mM aqueous sodium citrate solution (used as a stabilizer). A 37 mL sample of 22.4 mM aqueous NaBH4 solution was added dropwise under vigorous stirring, giving rise to a yellow Ag hydrosol. The Ag hydrosol was aged for 24 h to decompose the residual NaBH4 before it was used in subsequent steps. Third, the obtained Ag/ MWCNTs powder was dispersed in ethylene glycol (EG). Appropriate amounts of H2PtCl6$6H2O aqueous solution were added to a fiask. The pH of the system was adjusted to 7e8 with KOH/EG solution dropped. After the mixture was stirred for 4 h at 90 C, the Ag@Pt/MWCNTs electrocatalyst was collected. Ag@Pt/MWCNTs electrocatalysts with different mass ratios of Ag to Pt were synthesized according to the procedure described above by keeping the total metal loading at 40 wt%. For comparison, a Pt/MWCNTs catalyst with the same Pt content as Ag@Pt/MWCNTs was synthesized by using the method described above. Scheme 1 depicts the synthesis procedure for the Ag/MWCNTs nanomaterials and the subsequent deposition of a Pt layer. In this process, the oxygen functional groups (COOHe, OHe, COHe) on the surface of MWCNTs become active sites, which can absorb Agþ. Therefore, Ag nanoparticles can form easily and act as the core, and the Pt covering the Ag nanoparticles acts as the shell. The formation of coreeshell structure depends upon the metal salt solution and the reducing/stabilizing agent used during the preparation.
Scheme 1 e Schematic diagram for the formation of Ag@Pt/MWCNTs coreeshell nanostructure.
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2.2.
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Characterization
The morphology of the electrocatalyst was determined by high resolution transmission electron microscopy (HRTEM) on a JEOL S-520 microscope. X-ray diffraction (XRD) analysis was performed on a D/max-2200/PC X-ray diffractometer with a Cu Ka radiation source. The particle size was calculated by using the Scherrer equation [26]: D ¼ 0:9l=Bcos q
(1)
˚ , l is the X-ray wavewhere D is the diameter of particle in A length, q is the Bragg angle, and B is the full width at half maximum (FWHM, in radians) of the peak under consideration. Electrochemical measurements were carried out by using a Zahner Ennium electrochemical workstation with a threeelectrode cell. For cyclic voltammetry (CV) measurements and linear sweep voltammetry (LSV) measurements, a glassy carbon disk coated with electrocatalyst was used as the working electrode. The catalyst ink was produced by mixing 5 mg of the catalyst under sonication for 2 h in 0.1 ml Nafion solution and 0.9 ml ethanol. 5 ml or 10 ml of the electrocatalyst ink was dropped on the clean glassy carbon disk (3 mm or 5 mm outer diameter) and dried at room temperature. A Pt needle and a standard calomel electrode (SCE) were used as the counter-electrode and the reference electrode, respectively. The CV tests were conducted at 50 mV s1 in the potential range 0.2e1.2 V (vs. SCE). The LSV tests were conducted at 2 mV s1 in the potential range 0e0.8 V (vs. SCE).
3.
Results and discussion
Evidence for an AgePt coreeshell in Ag@Pt/MWCNTs can be seen in the XRD patterns in Fig. 1 (a). For comparison purposes, the XRD patterns for Ag/MWCNTs and Pt/MWCNTs prepared by a similar procedure are also shown in Fig. 1 (a). The peak located at about 2q ¼ 26.0 in all the XRD patterns is associated with the multi-walled carbon nanotube as supporter. The diffractions of Ag/MWCNTs (curve a in Fig. 1 (a)) matched well with those of the Ag characteristic peaks (2q ¼ 38.1, 44.2, and 64.4 ) (PDF card 87e0597). In curve b, the four peaks at 2q values of 39.8 , 46.2 , 67.4 and 81.3 are characteristics of the face-centered cubic (fcc) crystalline alloys of Pt (PDF card 04e0802), corresponding to the planes (111), (200), (220), and (311), respectively. In contrast, the 2q angles of characteristic diffractions peaks of Ag@Pt/MWCNTs as shown in curve c in Fig. 1 (a) were significantly lower than those of Pt/MWCNTs. The crystallinity of Ag is not obvious in Ag@Pt/MWCNTs, an indication that the Ag surface in Ag@Pt is mostly wrapped by Pt. This result is expected because the lattice constant for fcc Ag is larger than that for Pt [27]. According to the Bragg equation, the small negative shift of the Pt crystal characteristic peaks implies that the Pt lattice has expanded to cover the Ag surface, forming an Ag@Pt structure. The mean particle size of Ag@Pt nanoparticles was estimated by the Scherrer formula to be 14.2 nm in the Pt (111) lattice. The XRD results of Ag@Pt/MWCNTs catalysts with different mass ratios of Ag to Pt shown in Fig. 1 (b)
Fig. 1 e (a) XRD patterns of Ag/MWCNTs, Pt/MWCNTs, and Ag@Pt/MWCNTs catalysts. (b) XRD patterns of Ag@Pt/ MWCNTs catalysts with different Ag/Pt mass ratios at 40 wt% total metal. Insert figure: enlargement for characteristic diffraction peaks of Pt (111) and Pt (220) crystal surface.
demonstrate that with the decrease of Pt and increase of Ag, the extent of negative shift in the peak for Pt in Ag@Pt/ MWCNTs increases gradually. At the same time, the characteristic peaks for Pt becomes weaker, which confirms that the Pt lattice has contracted by loading on the Ag particles [28]. The insets in Fig. 1 (b) show the enlargement of the characteristic peaks of the plane (111) and (220), clearly indicating the negative shifts. Fig. 2 (a) shows that the catalysts after the loading of Pt are highly dispersed on the MWCNTs surface. Different from the faceted cubic or octahedral shape, the multi-walled carbon nanotubes supported metal (alloy) nanoparticles exhibited a spherical or elliptical shape. The coreeshell Ag@Pt particles with diameters from 12 to 17 nm were uniformly dispersed on the MWCNTs support. The lattice spacing of the Ag crystalline (111) plane is 0.236 nm, while the lattice spacing of the Pt crystalline (111) plane is 0.228 nm. As is shown in Fig. 2 (b), the coreeshell structure of nanoparticle is observed.
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Fig. 2 e (a) TEM image of Ag@Pt/MWCNTs catalyst. (b) HRTEM image of Ag@Pt catalyst.
Simultaneously, the lattice spacing of the PtAg crystalline (111) plane is 0.232 nm, which is between the lattice spacings of Pt and Ag, implying that the lattice spacing of Pt has expanded [18], in agreement with the XRD results shown in Fig. 1 There are two reasons for the formation of the coreeshell structure: first, the interaction between the metals is stronger than that between either metal and the carbon; second, both Pt crystallites and Ag crystallites have a facecentered cubic structure, a phenomenon in favor of the growth of Pt nanoparticles on the Ag surface [29]. The electrocatalytic activity of the Ag@Pt/MWCNTs catalysts was characterized by CV, and the results are shown in Fig. 3. Fig. 3 (a) shows the cyclic voltammograms of Ag@Pt/ MWCNTs and Pt/MWCNTs at same Pt loading. The catalysts exhibit H2 adsorption/desorption peaks in the potential range 0.2 to 0 V (vs. SCE). The Pt/MWCNTs scan exhibits two distinct potential regions, a typical voltammetric response of Pt surface under acidic conditions. In the hydrogen adsorption/desorption region, pure Pt shows two resolved peaks associated with weakly and strongly bonded hydrogen species on different crystal faces of Pt [30]. Furthermore, Ag@Pt/ MWCNTs exhibites well-defined current peaks associated with hydrogen adsorption-desorption processes. The cyclic voltammograms of Ag@Pt/MWCNTs coreeshell electrocatalysts with different mass ratios of Ag to Pt are shown in Fig. 3 (b). The electrochemical active surface area (EAS) of the catalysts was calculated according to Eq. (2) [31]: EAS m2 $g1 ¼ QH =ð2:1 ½PtÞ
(2)
2
where QH (C m ) is the charge exchanged during hydrogen desorption on the Pt surface and [Pt] (g$m2) is the Pt loading on the electrode. The calculated electrochemical active surface areas (EAS) of the catalysts are given in Table 1. According to Table 1, the EAS and MSA (Mass specific activity) of the H2 adsorption/desorption peaks are larger for Ag@Pt/ MWCNTs6 catalysts than for Pt/MWCNTs1 catalysts, indicating that the catalytic activity and utilization of Pt are better for coreeshell nanostructure catalysts than for pure Pt catalysts. The EAS of the Ag@Pt/MWCNTs4 catalyst with an Ag to Pt mass ratio of 10 wt% to 30 wt% was calculated to be
Fig. 3 e (a) Cyclic voltammograms of Ag@Pt/MWCNTs coreeshell electrocatalyst and Pt/MWCNTs catalyst with same Pt loading. (b) Cyclic voltammograms of Ag@Pt/ MWCNTs electrocatalysts with different Ag: Pt mass ratios. The electrolyte was 0.5 M H2SO4. Scanning rate was 50 mV sL1.
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Table 1 e Atomic Pt/Ag ratio, EAS, and MSA of Ag@Pt/MWCNTs electrocatalyst. Catalysts
Metal loading (wt %)
1
Pt/MWCNTs Pt/MWCNTs2 Ag@Pt/MWCNTs3 Ag@Pt/MWCNTs4 Ag@Pt/MWCNTs5 Ag@Pt/MWCNTs6
Pt
Ag
20 40 35 30 25 20
e e 5 10 15 20
Atomic Pt/Ag ratio
EAS (m2/g-Pt)
MSAa (mA/mg-Pt)
e e 3.89 1.66 0.92 0.55
31.18 40.96 55.59 70.63 34.73 33.54
42.0 49.9 63.9 56.3 60.0 51.8
a Mass specific activity of platinum at the peak potential (0.47 V vs. SCE) of oxygen reduction.
nanoparticles was about 14.2 nm, and the nanoparticles were well dispersed on the surface of MWCNT. The EAS of the Ag@Pt/MWCNTs catalyst was 70.63 m2 g1 at an Ag: Pt mass ratio of 10:30, and the atomic ratio Pt/Ag was 1.66. The mass content of Pt in the Ag@Pt/MWCNTs catalyst was lower than the value of 40% in the commercial Johnson-Matthey (JM) electrocatalyst. All the results show that the Ag@Pt/MWCNTs catalyst exhibits much higher catalytic activity than pure Pt catalyst and is very promising for PEMFC applications.
Acknowledgments
Fig. 4 e Polarization curves of Ag@Pt/MWCNTs and Pt/ MWCNTs catalysts in O2-saturated 0.5 mol LL1 H2SO4 at a rotation rate of 1600 rpm. Sweep rate: 2 mV sL1.
70.63 m2 g1. The MSA of all coreeshell electrocatalysts are large than that of pure Pt electrocatalysts. Fig. 4 shows that the onset reduction potential shifts 40 mV towards positive potential for the Ag@Pt/MWCNTs catalyst with respect to the corresponding pure Pt/MWCNTs catalyst. On the one hand, the surface oxide formation (Pt þ H2O / PteOH þ Hþ þ e) and the subsequent reduction of the surface oxide occur in the potential range [30]. This result shows that the oxygenated Pt species (PteOH species) have a weaker adsorption energy on the surface of the Ag@Pt/ MWCNTs catalyst than on the surface of the pure Pt catalyst, which means that the desorption of oxygenated Pt species is easier on Ag@Pt/MWCNTs than on the Pt/MWCNT [32]. The easier desorption on Ag@Pt/MWCNTs should facilitate the ORR and four-electron reduction reaction. On the other hand, the electrocatalytic activity enhancement for oxygen reduction is the result of the larger surface for Ag@Pt/MWCNTs, which provides more active sites.
4.
Conclusion
An Ag@Pt/MWCNTs electrocatalyst was synthesized by using NaBH4 and ethylene glycol as reducing agent and solvent, respectively. The average particle size of the Ag@Pt
The authors gratefully acknowledge the financial supports from the National Science Foundation of China (21176022), The International ration Program of China (No. 2009DFA63120), National High Technology Research and Development Program of China (No. 2011AA11A273 and No. 2012AA03A709) and National Defense Basic Scientific Research Program of China (No: A1420110023)
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