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Synthesis of three-dimensional Pd nanospheres decorated with a Pt monolayer for the oxygen reduction reaction
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Available online at www.sciencedirect.com
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Synthesis of three-dimensional Pd nanospheres decorated with a Pt monolayer for the oxygen reduction reaction Hualing Li, Shijun Liao*, Chenghang You, Bingqing Zhang, Huiyu Song, Zhiyong Fu Key Laboratory for Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China
article info
abstract
Article history:
Pd@Pt nanoparticles with a three-dimensional (3D) nanospherical shape have been pre-
Received 27 March 2014
pared using a two-step approach in which the Pd nanospheres are synthesized controllably
Received in revised form
by a solvothermal method, and then a Pt monolayer is deposited on the Pd nanospheres
13 June 2014
using an underpotential deposition process. We systematically investigate (i) the in-
Accepted 4 July 2014
fluences of temperature and additive on the morphology of the Pd nanoparticles and (ii) the
Available online 30 July 2014
electrochemical activity of Pd@Pt for the oxygen reduction reaction (ORR). The Pd@Pt nanoparticles exhibit enhanced activity towards the ORR. The mass activity of Pt in the
Keywords:
Pd@Pt nanospheres (1.03 A mgPt1) is 3.3 times higher than that of commercial Pt/C
Palladium
(0.24 A mgPt1). The catalyst enhanced activity may result from the 3D structure of the Pd
Platinum
nanospheres and the monolayer dispersion of Pt on the surface of the nanoparticles.
Three-dimensional nanosphere
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Underpotential deposition Oxygen reduction
Introduction Platinum nanoparticles supported on carbon materials are widely used as electrochemical catalysts for direct methanol fuel cells and proton exchange membrane fuel cells [1]. However, the high cost and poor durability mainly caused by using of precious and scarce platinum catalyst are two important obstacles to the commercialization of fuel cells [2,3]. The sluggish kinetics of the oxygen reduction reaction (ORR) is additional challenges [4]. Fuel cell researchers therefore urgently need to enhance electrochemical catalytic performance using the least amount of Pt, and to develop replacements for pure Pt catalysts.
A great deal of effort has been put into improving the ORR activity of Pt catalysts. A number of previous reports have shown that the use of a coreeshell catalyst with a Pt monolayer shell or ultrathin shell, in place of a pure Pt catalyst, can effectively increase platinum utilization [5,6]. The introduction of another metal as the core allows a much lower Pt loading and modifies the electronic and geometric structures of Pt, yielding enhanced activity. Numerous studies exploring various M@Pt coreeshell nanoparticles (M ¼ Pd, Au, Ag, Ni, and other metals) have shown that a Pd core has the greatest potential in the M@Pt structure to improve the ORR activity of Pt [7e9]. According to the results reported by Adzic et al. [10], the mass activity of Pt could be enhanced by over 4 times for
* Corresponding author. Fax: þ86 20 87113586. E-mail address: [email protected] (S. Liao). http://dx.doi.org/10.1016/j.ijhydene.2014.07.025 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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the coreeshell nanoparticles with Pd nanoparticles as core and with a Pt monolayer of shell. As is widely known, electrocatalytic activity is not only determined by the composition of the electrocatalyst but also by the morphology of the nanoparticles [11]. For example, the activity of 7 nm Pt nanocubes is 4 times that of 3 nm polyhedral Pt nanoparticles for the ORR [11]. Pt concave nanocubes enclosed by high-index facets exhibit substantially enhanced electrocatalytic activity per unit surface area towards the ORR, compared with Pt cubes and commercial Pt/C [12]. In previous studies, Pt nanocrystals with various morphologies e such as polyhedrons [13,14], mutipods [15], nanowires [16], and nanoflowers [17] with three-dimensional (3D) dendritic or flower structures e have been synthesized. Interestingly, 3D dendritic Pt has attracted the special attention of researchers for its high surface-to-volume ratio and sufficient adsorption sites for all participating molecules in a limited space. On the basis of this research background, Pd@Pt nanoparticles with 3D structures are expected to exhibit excellent performance for the ORR. There are couples of additional works preparing the Pd@Pt nanoparticles [18e20], but in this study, we synthesized 3D Pd nanospheres, composed of fine nanostructures on the surface of the described nanospheres, using a solvothermal approach. We then investigated the effects of temperature and additives on the nanospheres' morphology and properties. We also deposited a Pt monolayer (Pt ML) onto the surface of the asprepared Pd nanospheres using an underpotential deposition method, in which a Cu monolayer was deposited first, followed by galvanic replacement of the Cu monolayer with Pt, yielding a coreeshell Pd@Pt ML nanoparticle. This Pd@Pt nanoparticle exhibited better performance towards the ORR than that of commercial Pt/C catalyst, confirming the superiority of this type of specially structured nanoparticle.
Experimental Synthesis of 3D Pd nanospheres In a typical procedure, 0.2 mmol of PdCl2, 80.0 mL of benzyl alcohol, 2.0 mL of oleylamine (OAm), and 2.0 mL of oleic acid (OA) were loaded into a 100 mL beaker under stirring. The volume ratio of OAm to OA was 1:1. The mixture was then transferred to a stainless steel autoclave lined with a Teflon vessel (inner volume 100 mL) and heated at 120 C for 8 h, followed by cooling to room temperature, centrifuging at 10,000 rpm, and washing with ethanol three times to remove the residual surfactant.
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surface of the Pd nanoparticles via galvanic displacement between Pt2þ and Cu, by immersing the electrode in a solution of 1 mM K2PtCl4 þ 50 mM H2SO4. Unless otherwise specified, all the Pd nanosphere samples in this paper were synthesized at 120 C for 8 h and with an OA/OAm ratio of 1:1 (by volume). All of the Pd@Pt samples were prepared based on the above described Pd nanoparticles, and the calculated the content of Pt in the prepared Pd@Pt coreeshell nanoparticles could be calculated from the depositing charges.
Characterization X-ray powder diffraction (XRD) was conducted on a TD-3500 powder diffractometer (Tongda, China) operated at 40 kV and 30 mA, using Cu-Ka radiation. X-ray photoelectron spectroscopy (XPS) was performed on an X-ray photoelectron spectrometer (Kratos, England) employing a monochromated AleKa X-ray source (hn ¼ 1486.6 eV). Scanning electron microscopy (SEM) images were recorded on Hitachi S-3700 and Nova NanoSEM 430 microscopes. High-resolution transmission electron microscopy (HRTEM) images were recorded on a JEM-2100 HR microscope (JEOL, Japan) operated at 200 kV.
Performance measurement The ORR performance measurements were conducted on a standard three-electrode electrochemical system equipped with a gas flow system, and an electrochemical workstation (Ivium, Netherlands) at room temperature, coupled with a rotating disk electrode (RDE) system (PINE, USA). An Ag/AgCl electrode (saturated, KCl filled) as the reference electrode and a Pt wire as the counter electrode, the electrode prepared with UPD method, loaded of Pd and deposited with Pt monolayer of shell, was used as working electrode, the calculated loading amount of Pd nanospheres is 50 mg, and the calculated loading of Pt is 2.83 mg. The ORR polarization curves was measured at a scan rate of 10 mV s1 and with a disk rotating rate of 1600 rpm in O2-saturated 0.1 M HClO4 solution. Chronoamperometry method was used to test the durability of the catalysts. It is conducted at potential of 0.4 V, a disk rotating rate of 900 rpm, and in O2-saturated 0.1 M HClO4 solution. The stability of the catalysts was investigated by a potential cycling from 0.4 V to 0.8 V (vs. Ag/AgCl) at temperature of 70 C in air-saturated 0.1 M HClO4 solution.
Results and discussion SEM and TEM images of Pd nanospheres
Preparation of 3D Pd@Pt nanospheres by underpotential deposition 5 mg of the synthesized 3D Pd nanospheres were dispersed ultrasonically in 1 mL Nafion/ethanol (0.05 wt% Nafion) for 30 min, then 10 mL of the Pd nanosphere dispersion were pipetted and spread on a glassy carbon electrode (GCE) and dried in air. Next, deposition of the Pt ML was performed using an underpotential deposition (UPD) method [21]: a Cu monolayer was deposited on the Pd nanospheres, then a Pt monolayer was formed on the
Fig. 1a, b, and c show SEM images of the synthesized Pd nanoparticles at various magnifications. They are uniform spherical structures with diameters of about 300e400 nm and consist of dendritic nanoparticles that are densely dispersed in an orderly fashion on the surface of the nanospheres e i.e., the Pd nanospheres have a 3D dendritic structure. The TEM image shown in Fig. 1d further confirms their 3D structure. The surface of the Pd nanospheres is comprised of nanosheets with an average length of 10e15 nm.
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Fig. 1 e SEM images of Pd nanospheres: (a) overall morphology; (b) and (c) higher magnification images of Pd nanospheres; (d) TEM image of a Pd nanosphere synthesized at 120 C for 8 h with OA/OAm (1:1) as additives.
Effect of synthesis temperature Fig. 2a shows the XRD patterns of Pd particles synthesized at 120 C, 140 C, and 160 C. The sharp diffraction peaks located at 40.1, 46.6, 68.1, 82.1, and 86.6 are assigned to the (111), (200),
(220), (311), and (222) crystal facets of Pd, respectively, indicating that the Pd particles have a face-centered cubic crystal structure (JCPDS standard 05-0681). No other impurity peaks are observed in the XRD patterns, suggesting that the Pd particles have high purity and good crystallinity. By using
Fig. 2 e (a) XRD patterns of Pd particles synthesized at 120, 140, and 160 C; (b), (c), and (d) SEM images of Pd particles synthesized at 120, 140, and 160 C, with an OA/OAm composite additive (OA/OAm ¼ 1).
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Scherrer's equation, the average crystallite sizes of the primary particles of the Pd nanospheres synthesized at 120 C, 140 C, and 160 C are 8.3, 9.0, and 8.3 nm, respectively. There seems no obvious difference in the crystallite size for three samples. However, as shown in Fig. 2b, c and d, we can see that higher temperature will result in larger particle size and uneven particle size distribution.
Effect of synthesis additives We found that the addition of OA and OAm as additives played a key role in the formation of nanospheres with fine nanostructures. Fig. 3 shows SEM images of Pd nanostructures synthesized with various additives. When no OA or OAm was added, the Pd nanoparticles formed irregular shapes with sizes of 100e200 nm (Fig. 3a). However, when pure OAm was added, the shape of the Pd nanoparticles changed dramatically (Fig. 3b); they were spherical, and some dendritic nanoparticles were distributed on the surface. Clearly, OAm plays an important role in the formation of Pd nanospheres. We also investigated the effects of an OA/OAm composite additive on the structure of the Pd nanoparticles. Adding an
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OA/OAm ¼ 1 composite additive resulted in more dendritic structures on the spherical surface (Fig. 3c and d) than adding only OAm (Fig. 3b). Once the ratio of OA to OAm was decreased to 1:2, almost no morphological change was observable (Fig. 3e) as compared to Pd nanospheres synthesized with pure OAm (Fig. 3b). However, when the ratio was increased to 2:1, the morphology changed remarkably, becoming smaller and irregular (Fig. 3f). Clearly, 1:1 is the optimal ratio of OA to OAm to form the dendritic surface structure and spherical morphology. The addition of just OAm can result in good spherical morphology (Fig. 3b); however, as will be discussed later, the Pd@Pt catalyst thus derived exhibited worse ORR activity than the catalyst created with a composite additive of OA and OAm, indicating the significance of the dendritic surface structure. Based on the above results, we suggest a possible formation mechanism for the synthesis of our spherical Pd nanoparticles with dendric structures (Scheme 1), the OA and OAm play as surfactants/template, and the benzyl alcohol was used as solvent and reductant. Firstly, the OA and OAm were dissolved in benzyl alcohol to form spherical micelles with dendric outer structure, then the Pd2þ ions were slowly reduced
Fig. 3 e SEM images of Pd nanoparticles: (a) no addition of OA or OAm; (b) addition of pure OAm; (c) and (d) 1:1 ratio of OA to OAm; (e) 1:2 ratio of OA to OAm; (f) 2:1 ratio of OA to OAm.
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Scheme 1 e Schematic illustration of the proposed growth mechanism for Pd nanoparticles.
by benzyl alcohol and deposited into the inside of micelles and on the dendrics, finally, the Pd nanoparticles with special morphology were obtained by removing the OA/OAm templates with ethanol washing. By adjusting the ratio of OA/ OAm, or changing the reducing temperature, the nanoparticles with various morphologies could be synthesized.
Characterization of Pd@Pt nanospheres The HRTEM image in Fig. 4a clearly reveals that the deposited platinum nanoparticles still retain a spherical structure. Fig. 4b shows the morphology of a selected sphere. Numerous branches on the Pd@Pt spherical surface can be observed. The structural characteristics of these Pd@Pt nanospheres were further confirmed by high-angle annular dark-field scanning TEM (HAADF-STEM) (Fig. 5a). Their chemical composition was determined by corresponding energy-dispersive X-ray spectroscopy (EDS) (Fig. 5b). In EDS, only the peaks for Pd and Pt were observed (except for copper signals from the TEM grid), demonstrating that the Pd@Pt nanospheres were composed exclusively of palladium and platinum. As presented in Fig. 5c, d and e, we characterized the Pd@Pt nanoparticles using SEM-EDS mapping. This technique showed that Pt was highly distributed, with a low density, throughout the Pd nanospherical surface (Fig. 5e). In contrast, the Pd density was very high, confirming the coreeshell structure of the Pd@Pt nanospheres prepared by the underpotential deposition method. Fig. 6b shows the XPS spectrum of Pd 3d of our prepared Pd@Pt coreeshell catalyst, by analyzed with deconvolution,
the peaks with binding energies of 340.14 eV and 334.82 eV were ascribed to Pd 3d3/2 and Pd 3d5/2 of Pd0. The peaks negatively shifted 0.31 eV and 0.24 eV compared with the results for Pd in Pd nanosphere as shown in Fig. 6a. Fig. 6d shows the XPS spectrum of Pt 4f of Pd@Pt coreeshell catalyst, and the binding energies of Pt 4f7/2 and Pt 4f5/2 of Pt0 are 71.53 eV and 74.97 eV, respectively, by the deconvolution results. Compared with the results of Pt/C shown in Fig. 6c, the binding energies of Pt 4f7/2 and Pt 4f5/2 shifted 0.33 eV and 0.35 eV. The negative shifts in Pd 3d binding energy, as well as the positive shifts in Pt 4f, could be attributed to the presence of a Pt monolayer on the surface of the Pd nanoparticles, and to electronic interaction between the Pt shell and the Pd core.
Oxygen reduction reaction As shown in Fig. 7a, the Pd nanospheres exhibited poor ORR activity; however, after the Pt monolayer was deposited, the Pd@Pt nanoparticles exhibited excellent activity, superior to that of commercial 20% Pt/C. The onset potential was 0.79 V (measured at 0.01 mA cm2) and the half-wave potential was 0.64 V; the current density at 0.6 V (vs. Ag/AgCl) was 4.2 mA cm2, which was 3.2 times the current density of the Pd nanospheres and 1.4 times that of commercial 20% Pt/C, indicating the greatly enhanced performance of the coreeshell Pd@Pt nanospheres. We suggest that the enhancement is due to two factors: the monolayer deposition of Pt, and the interaction between this Pt in the shell and the Pd in the core. It should be pointed out that the mass activity presented in Fig. 7c also confirms this greatly enhanced activity. The mass
Fig. 4 e HRTEM images of Pd@Pt spheres: (a) overall morphology; (b) high-magnification image of a sphere.
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Fig. 5 e (a) HAADF-STEM image of a Pd@Pt nanosphere; (b) energy-dispersive X-ray analysis of a Pd@Pt nanosphere; (c) SEM image of Pd@Pt nanospheres; (d) and (e) corresponding SEM-EDS mapping of Pd and Pt, respectively.
Fig. 6 e (a) The Pd 3d XPS spectrum of Pd nanopartilce; (b) the Pd 3d XPS spectrum of Pd@Pt nanoparticles; (c) the Pt 4f XPS spectrum of 20% Pt/C; (d) the Pt 4f XPS spectrum of Pd@Pt nanoparticles.
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Fig. 7 e (a) ORR curves for Pd nanospheres, 20% Pt/C, and Pd@Pt nanospheres; (b) ORR curves for a series of Pd@Pt nanoparticles based on Pd nanospheres synthesized using composite additives with various ratios of OA/OAm; (c) column diagram of the mass activity of Pt for several catalysts with different Pd nanoparticles as the core; (d) the cyclic voltammograms (CVs) of various catalysts in N2-saturated 0.5 M H2SO4 solution.
activity of Pt for Pd@Pt core shell catalyst reached 1.03 A mgPt1 e 3.3 times higher than that of commercial 20% Pt/C. By depositing a Pt monolayer on the surface of various Pd nanoparticles synthesized with different additives, we prepared a series of Pd@Pt core shell nanoparticles. We found that the additives used for the synthesis of Pd nanoparticles significantly affected the performance of the final Pd@Pt coreeshell catalysts. Fig. 7b shows the initial polarization curves of these catalysts towards the ORR. We also used column graphs to present the mass activities of Pt for various Pd@Pt nanoparticles (Fig. 7c). Combining the results shown in Fig. 7b and c, we can see that the Pd@Pt with an OA/OAm ratio of 1 clearly exhibited higher ORR performance than the other two samples, with OA/OAm ratios of 2:1 and 1:2, respectively. Fig. 7d shows the cyclic voltammograms (CVs) of Pd nanosphere, Pt/C and various Pd@Pt nanoparticles in N2saturated 0.5 M H2SO4 solution, from these CV curves, we can see that Pd nanospheres show much larger peak areas than the Pt/C and various Pd@Pt nanoparticles for hydrogen adsorption/desorption, it may be ascribed to the high surface area of flower-like Pd nanoparticles, and the strong adsorption of Pd to the hydrogen. We can calculate the electrochemical active surface area (ECSA) for various Pd@Pt nanoparticles prepared with various templates composition. Furthermore, based on the assumption of monolayer dispersion of Pt, we obtained the Pt amount deposited on the surface of Pd nanospheres for each samples, and we assembled these results in Table 1. From Table 1, we can see the effects of OA and OAm additives on the performance of Pd@Pt coreeshell catalysts. With
the increase of the ratio of OA/OAm the ECSA and Pt loading are increased significantly, however, the catalyst with the OA/OAm ratio of 1/1 exhibits highest ORR performance (kinetic current). We suggest that the best performance of the catalyst may be result from its optimal morphology of the Pd nanospheres used as the core for the catalyst, and the OA/OAm ratio of 1 is an optimal ratio for the formation of desired morphology. Frankly, for the effect of OA/OAm ratio on the performance of final coreeshell catalysts, it needs further investigation. The durability of ORR electrocatalysts is one of the key issues for catalyst used for fuel cells. We therefore compared the electrochemical stabilities of these Pd@Pt core shell catalysts with the stability of commercial 20% Pt/C catalyst at room temperature and at high temperature of 70 C. At room temperature, the stabilities of both catalysts were compared by a chronoamperometry method, the Pd@Pt
Table 1 e Pt loading and electrochemical information from Pd@Pt nanoparticles based on Pd nanospheres synthesized using composite additives with various volume ratio of OA/OAm. Ratio of Electrochemical Pt loading/mg Kinetic current OA/OAm active specific at 0.6 area (ECSA)/cm2 V/mA vs. Ag/AgCl 0/1 1/2 1/1 2/1 Pt/C
1.22 2.25 5.12 5.04 1.22
0.92 1.98 2.83 3.80 5
0.23 0.85 2.89 1.22 1.19
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Fig. 8 e (a) Chronoamperometric tests results for 20% Pt/C catalyst and Pd@Pt nanospheres at 0.4 V for 30,000 s with a rotation rate of 900 rpm; the ORR polarization curves of Pd@Pt nanospheres (b) and 20% Pt/C (c) before and after the 10,000 cycles; column diagrams of the relative kinetic current at 0.6 V of Pd@Pt nanospheres and 20% Pt/C were inserted in (b) and (c).
catalyst presents better stability than that of 20% Pt/C catalyst. As shown in Fig. 8a, the current density loss at 0.4 V is 34% after 30,000 s for Pd@Pt catalyst, but 48% for commercial Pt/C catalyst. At high temperature of 70 C, the stability of the catalysts was investigated by a potential cycling from 0.4 V to 0.8 V (vs. Ag/AgCl) in 0.1 M HClO4 solution for 10,000 cycles. The both catalysts show almost same losses in onset potential. The Pd@Pt nanospheres show 45-mV degradation in half-wave potential after 10,000 cycles (Fig. 8b), while the degradation for 20% Pt/C is 55 mV (Fig. 8c). The Pd@Pt core shell catalyst shows the loss of 75.8% in kinetic current at 0.6 V (vs. Ag/AgCl) after 10,000 cycles (Fig. 8b). And the 20% Pt/C catalyst shows a loss of 83.2% of the kinetic current after 10,000 cycles (Fig. 8c). All the test results at room temperature and high temperature confirm the stability of Pd@Pt catalyst is a little better. Actually there is no big difference between them.
solvothermal method and then covered with a Pt monolayer using an underpotential deposition method. The coreeshell nanospheres exhibited substantially enhanced ORR activity compared to 20% Pt/C, and their Pt mass activity was 3.3 times higher than that of 20% Pt/C. It is suggested that this significant performance enhancement may be attributable to the nanoparticles' 3D dendritic morphology and the interactions between the Pt in the shell and the Pd in the core.
Acknowledgments This work is supported by the National Foundation of China (NFC Project NO. 21276098), the Nature Scientific Foundation of Guangdong Province, China (NSFGC Project NO. S2012020011061), and the Science and Technology 863 Foundation (STF Project NO. 2012AA053402).
Conclusion Appendix A. Supplementary data Pd@Pt nanoparticles with 3D nanospherical morphology and dendritic surface nanostructure have been prepared by a twostep approach. 3D Pd nanospheres were synthesized using a
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.07.025.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Synthesis of three-dimensional Pd nanospheres decorated with a Pt monolayer for the oxygen reduction reaction Hualing Li, Shijun Liao*, Chenghang You, Bingqing Zhang, Huiyu Song, Zhiyong Fu Key Laboratory for Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China
article info
abstract
Article history:
Pd@Pt nanoparticles with a three-dimensional (3D) nanospherical shape have been pre-
Received 27 March 2014
pared using a two-step approach in which the Pd nanospheres are synthesized controllably
Received in revised form
by a solvothermal method, and then a Pt monolayer is deposited on the Pd nanospheres
13 June 2014
using an underpotential deposition process. We systematically investigate (i) the in-
Accepted 4 July 2014
fluences of temperature and additive on the morphology of the Pd nanoparticles and (ii) the
Available online 30 July 2014
electrochemical activity of Pd@Pt for the oxygen reduction reaction (ORR). The Pd@Pt nanoparticles exhibit enhanced activity towards the ORR. The mass activity of Pt in the
Keywords:
Pd@Pt nanospheres (1.03 A mgPt1) is 3.3 times higher than that of commercial Pt/C
Palladium
(0.24 A mgPt1). The catalyst enhanced activity may result from the 3D structure of the Pd
Platinum
nanospheres and the monolayer dispersion of Pt on the surface of the nanoparticles.
Three-dimensional nanosphere
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Underpotential deposition Oxygen reduction
Introduction Platinum nanoparticles supported on carbon materials are widely used as electrochemical catalysts for direct methanol fuel cells and proton exchange membrane fuel cells [1]. However, the high cost and poor durability mainly caused by using of precious and scarce platinum catalyst are two important obstacles to the commercialization of fuel cells [2,3]. The sluggish kinetics of the oxygen reduction reaction (ORR) is additional challenges [4]. Fuel cell researchers therefore urgently need to enhance electrochemical catalytic performance using the least amount of Pt, and to develop replacements for pure Pt catalysts.
A great deal of effort has been put into improving the ORR activity of Pt catalysts. A number of previous reports have shown that the use of a coreeshell catalyst with a Pt monolayer shell or ultrathin shell, in place of a pure Pt catalyst, can effectively increase platinum utilization [5,6]. The introduction of another metal as the core allows a much lower Pt loading and modifies the electronic and geometric structures of Pt, yielding enhanced activity. Numerous studies exploring various M@Pt coreeshell nanoparticles (M ¼ Pd, Au, Ag, Ni, and other metals) have shown that a Pd core has the greatest potential in the M@Pt structure to improve the ORR activity of Pt [7e9]. According to the results reported by Adzic et al. [10], the mass activity of Pt could be enhanced by over 4 times for
* Corresponding author. Fax: þ86 20 87113586. E-mail address: [email protected] (S. Liao). http://dx.doi.org/10.1016/j.ijhydene.2014.07.025 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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the coreeshell nanoparticles with Pd nanoparticles as core and with a Pt monolayer of shell. As is widely known, electrocatalytic activity is not only determined by the composition of the electrocatalyst but also by the morphology of the nanoparticles [11]. For example, the activity of 7 nm Pt nanocubes is 4 times that of 3 nm polyhedral Pt nanoparticles for the ORR [11]. Pt concave nanocubes enclosed by high-index facets exhibit substantially enhanced electrocatalytic activity per unit surface area towards the ORR, compared with Pt cubes and commercial Pt/C [12]. In previous studies, Pt nanocrystals with various morphologies e such as polyhedrons [13,14], mutipods [15], nanowires [16], and nanoflowers [17] with three-dimensional (3D) dendritic or flower structures e have been synthesized. Interestingly, 3D dendritic Pt has attracted the special attention of researchers for its high surface-to-volume ratio and sufficient adsorption sites for all participating molecules in a limited space. On the basis of this research background, Pd@Pt nanoparticles with 3D structures are expected to exhibit excellent performance for the ORR. There are couples of additional works preparing the Pd@Pt nanoparticles [18e20], but in this study, we synthesized 3D Pd nanospheres, composed of fine nanostructures on the surface of the described nanospheres, using a solvothermal approach. We then investigated the effects of temperature and additives on the nanospheres' morphology and properties. We also deposited a Pt monolayer (Pt ML) onto the surface of the asprepared Pd nanospheres using an underpotential deposition method, in which a Cu monolayer was deposited first, followed by galvanic replacement of the Cu monolayer with Pt, yielding a coreeshell Pd@Pt ML nanoparticle. This Pd@Pt nanoparticle exhibited better performance towards the ORR than that of commercial Pt/C catalyst, confirming the superiority of this type of specially structured nanoparticle.
Experimental Synthesis of 3D Pd nanospheres In a typical procedure, 0.2 mmol of PdCl2, 80.0 mL of benzyl alcohol, 2.0 mL of oleylamine (OAm), and 2.0 mL of oleic acid (OA) were loaded into a 100 mL beaker under stirring. The volume ratio of OAm to OA was 1:1. The mixture was then transferred to a stainless steel autoclave lined with a Teflon vessel (inner volume 100 mL) and heated at 120 C for 8 h, followed by cooling to room temperature, centrifuging at 10,000 rpm, and washing with ethanol three times to remove the residual surfactant.
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surface of the Pd nanoparticles via galvanic displacement between Pt2þ and Cu, by immersing the electrode in a solution of 1 mM K2PtCl4 þ 50 mM H2SO4. Unless otherwise specified, all the Pd nanosphere samples in this paper were synthesized at 120 C for 8 h and with an OA/OAm ratio of 1:1 (by volume). All of the Pd@Pt samples were prepared based on the above described Pd nanoparticles, and the calculated the content of Pt in the prepared Pd@Pt coreeshell nanoparticles could be calculated from the depositing charges.
Characterization X-ray powder diffraction (XRD) was conducted on a TD-3500 powder diffractometer (Tongda, China) operated at 40 kV and 30 mA, using Cu-Ka radiation. X-ray photoelectron spectroscopy (XPS) was performed on an X-ray photoelectron spectrometer (Kratos, England) employing a monochromated AleKa X-ray source (hn ¼ 1486.6 eV). Scanning electron microscopy (SEM) images were recorded on Hitachi S-3700 and Nova NanoSEM 430 microscopes. High-resolution transmission electron microscopy (HRTEM) images were recorded on a JEM-2100 HR microscope (JEOL, Japan) operated at 200 kV.
Performance measurement The ORR performance measurements were conducted on a standard three-electrode electrochemical system equipped with a gas flow system, and an electrochemical workstation (Ivium, Netherlands) at room temperature, coupled with a rotating disk electrode (RDE) system (PINE, USA). An Ag/AgCl electrode (saturated, KCl filled) as the reference electrode and a Pt wire as the counter electrode, the electrode prepared with UPD method, loaded of Pd and deposited with Pt monolayer of shell, was used as working electrode, the calculated loading amount of Pd nanospheres is 50 mg, and the calculated loading of Pt is 2.83 mg. The ORR polarization curves was measured at a scan rate of 10 mV s1 and with a disk rotating rate of 1600 rpm in O2-saturated 0.1 M HClO4 solution. Chronoamperometry method was used to test the durability of the catalysts. It is conducted at potential of 0.4 V, a disk rotating rate of 900 rpm, and in O2-saturated 0.1 M HClO4 solution. The stability of the catalysts was investigated by a potential cycling from 0.4 V to 0.8 V (vs. Ag/AgCl) at temperature of 70 C in air-saturated 0.1 M HClO4 solution.
Results and discussion SEM and TEM images of Pd nanospheres
Preparation of 3D Pd@Pt nanospheres by underpotential deposition 5 mg of the synthesized 3D Pd nanospheres were dispersed ultrasonically in 1 mL Nafion/ethanol (0.05 wt% Nafion) for 30 min, then 10 mL of the Pd nanosphere dispersion were pipetted and spread on a glassy carbon electrode (GCE) and dried in air. Next, deposition of the Pt ML was performed using an underpotential deposition (UPD) method [21]: a Cu monolayer was deposited on the Pd nanospheres, then a Pt monolayer was formed on the
Fig. 1a, b, and c show SEM images of the synthesized Pd nanoparticles at various magnifications. They are uniform spherical structures with diameters of about 300e400 nm and consist of dendritic nanoparticles that are densely dispersed in an orderly fashion on the surface of the nanospheres e i.e., the Pd nanospheres have a 3D dendritic structure. The TEM image shown in Fig. 1d further confirms their 3D structure. The surface of the Pd nanospheres is comprised of nanosheets with an average length of 10e15 nm.
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Fig. 1 e SEM images of Pd nanospheres: (a) overall morphology; (b) and (c) higher magnification images of Pd nanospheres; (d) TEM image of a Pd nanosphere synthesized at 120 C for 8 h with OA/OAm (1:1) as additives.
Effect of synthesis temperature Fig. 2a shows the XRD patterns of Pd particles synthesized at 120 C, 140 C, and 160 C. The sharp diffraction peaks located at 40.1, 46.6, 68.1, 82.1, and 86.6 are assigned to the (111), (200),
(220), (311), and (222) crystal facets of Pd, respectively, indicating that the Pd particles have a face-centered cubic crystal structure (JCPDS standard 05-0681). No other impurity peaks are observed in the XRD patterns, suggesting that the Pd particles have high purity and good crystallinity. By using
Fig. 2 e (a) XRD patterns of Pd particles synthesized at 120, 140, and 160 C; (b), (c), and (d) SEM images of Pd particles synthesized at 120, 140, and 160 C, with an OA/OAm composite additive (OA/OAm ¼ 1).
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Scherrer's equation, the average crystallite sizes of the primary particles of the Pd nanospheres synthesized at 120 C, 140 C, and 160 C are 8.3, 9.0, and 8.3 nm, respectively. There seems no obvious difference in the crystallite size for three samples. However, as shown in Fig. 2b, c and d, we can see that higher temperature will result in larger particle size and uneven particle size distribution.
Effect of synthesis additives We found that the addition of OA and OAm as additives played a key role in the formation of nanospheres with fine nanostructures. Fig. 3 shows SEM images of Pd nanostructures synthesized with various additives. When no OA or OAm was added, the Pd nanoparticles formed irregular shapes with sizes of 100e200 nm (Fig. 3a). However, when pure OAm was added, the shape of the Pd nanoparticles changed dramatically (Fig. 3b); they were spherical, and some dendritic nanoparticles were distributed on the surface. Clearly, OAm plays an important role in the formation of Pd nanospheres. We also investigated the effects of an OA/OAm composite additive on the structure of the Pd nanoparticles. Adding an
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OA/OAm ¼ 1 composite additive resulted in more dendritic structures on the spherical surface (Fig. 3c and d) than adding only OAm (Fig. 3b). Once the ratio of OA to OAm was decreased to 1:2, almost no morphological change was observable (Fig. 3e) as compared to Pd nanospheres synthesized with pure OAm (Fig. 3b). However, when the ratio was increased to 2:1, the morphology changed remarkably, becoming smaller and irregular (Fig. 3f). Clearly, 1:1 is the optimal ratio of OA to OAm to form the dendritic surface structure and spherical morphology. The addition of just OAm can result in good spherical morphology (Fig. 3b); however, as will be discussed later, the Pd@Pt catalyst thus derived exhibited worse ORR activity than the catalyst created with a composite additive of OA and OAm, indicating the significance of the dendritic surface structure. Based on the above results, we suggest a possible formation mechanism for the synthesis of our spherical Pd nanoparticles with dendric structures (Scheme 1), the OA and OAm play as surfactants/template, and the benzyl alcohol was used as solvent and reductant. Firstly, the OA and OAm were dissolved in benzyl alcohol to form spherical micelles with dendric outer structure, then the Pd2þ ions were slowly reduced
Fig. 3 e SEM images of Pd nanoparticles: (a) no addition of OA or OAm; (b) addition of pure OAm; (c) and (d) 1:1 ratio of OA to OAm; (e) 1:2 ratio of OA to OAm; (f) 2:1 ratio of OA to OAm.
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Scheme 1 e Schematic illustration of the proposed growth mechanism for Pd nanoparticles.
by benzyl alcohol and deposited into the inside of micelles and on the dendrics, finally, the Pd nanoparticles with special morphology were obtained by removing the OA/OAm templates with ethanol washing. By adjusting the ratio of OA/ OAm, or changing the reducing temperature, the nanoparticles with various morphologies could be synthesized.
Characterization of Pd@Pt nanospheres The HRTEM image in Fig. 4a clearly reveals that the deposited platinum nanoparticles still retain a spherical structure. Fig. 4b shows the morphology of a selected sphere. Numerous branches on the Pd@Pt spherical surface can be observed. The structural characteristics of these Pd@Pt nanospheres were further confirmed by high-angle annular dark-field scanning TEM (HAADF-STEM) (Fig. 5a). Their chemical composition was determined by corresponding energy-dispersive X-ray spectroscopy (EDS) (Fig. 5b). In EDS, only the peaks for Pd and Pt were observed (except for copper signals from the TEM grid), demonstrating that the Pd@Pt nanospheres were composed exclusively of palladium and platinum. As presented in Fig. 5c, d and e, we characterized the Pd@Pt nanoparticles using SEM-EDS mapping. This technique showed that Pt was highly distributed, with a low density, throughout the Pd nanospherical surface (Fig. 5e). In contrast, the Pd density was very high, confirming the coreeshell structure of the Pd@Pt nanospheres prepared by the underpotential deposition method. Fig. 6b shows the XPS spectrum of Pd 3d of our prepared Pd@Pt coreeshell catalyst, by analyzed with deconvolution,
the peaks with binding energies of 340.14 eV and 334.82 eV were ascribed to Pd 3d3/2 and Pd 3d5/2 of Pd0. The peaks negatively shifted 0.31 eV and 0.24 eV compared with the results for Pd in Pd nanosphere as shown in Fig. 6a. Fig. 6d shows the XPS spectrum of Pt 4f of Pd@Pt coreeshell catalyst, and the binding energies of Pt 4f7/2 and Pt 4f5/2 of Pt0 are 71.53 eV and 74.97 eV, respectively, by the deconvolution results. Compared with the results of Pt/C shown in Fig. 6c, the binding energies of Pt 4f7/2 and Pt 4f5/2 shifted 0.33 eV and 0.35 eV. The negative shifts in Pd 3d binding energy, as well as the positive shifts in Pt 4f, could be attributed to the presence of a Pt monolayer on the surface of the Pd nanoparticles, and to electronic interaction between the Pt shell and the Pd core.
Oxygen reduction reaction As shown in Fig. 7a, the Pd nanospheres exhibited poor ORR activity; however, after the Pt monolayer was deposited, the Pd@Pt nanoparticles exhibited excellent activity, superior to that of commercial 20% Pt/C. The onset potential was 0.79 V (measured at 0.01 mA cm2) and the half-wave potential was 0.64 V; the current density at 0.6 V (vs. Ag/AgCl) was 4.2 mA cm2, which was 3.2 times the current density of the Pd nanospheres and 1.4 times that of commercial 20% Pt/C, indicating the greatly enhanced performance of the coreeshell Pd@Pt nanospheres. We suggest that the enhancement is due to two factors: the monolayer deposition of Pt, and the interaction between this Pt in the shell and the Pd in the core. It should be pointed out that the mass activity presented in Fig. 7c also confirms this greatly enhanced activity. The mass
Fig. 4 e HRTEM images of Pd@Pt spheres: (a) overall morphology; (b) high-magnification image of a sphere.
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Fig. 5 e (a) HAADF-STEM image of a Pd@Pt nanosphere; (b) energy-dispersive X-ray analysis of a Pd@Pt nanosphere; (c) SEM image of Pd@Pt nanospheres; (d) and (e) corresponding SEM-EDS mapping of Pd and Pt, respectively.
Fig. 6 e (a) The Pd 3d XPS spectrum of Pd nanopartilce; (b) the Pd 3d XPS spectrum of Pd@Pt nanoparticles; (c) the Pt 4f XPS spectrum of 20% Pt/C; (d) the Pt 4f XPS spectrum of Pd@Pt nanoparticles.
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Fig. 7 e (a) ORR curves for Pd nanospheres, 20% Pt/C, and Pd@Pt nanospheres; (b) ORR curves for a series of Pd@Pt nanoparticles based on Pd nanospheres synthesized using composite additives with various ratios of OA/OAm; (c) column diagram of the mass activity of Pt for several catalysts with different Pd nanoparticles as the core; (d) the cyclic voltammograms (CVs) of various catalysts in N2-saturated 0.5 M H2SO4 solution.
activity of Pt for Pd@Pt core shell catalyst reached 1.03 A mgPt1 e 3.3 times higher than that of commercial 20% Pt/C. By depositing a Pt monolayer on the surface of various Pd nanoparticles synthesized with different additives, we prepared a series of Pd@Pt core shell nanoparticles. We found that the additives used for the synthesis of Pd nanoparticles significantly affected the performance of the final Pd@Pt coreeshell catalysts. Fig. 7b shows the initial polarization curves of these catalysts towards the ORR. We also used column graphs to present the mass activities of Pt for various Pd@Pt nanoparticles (Fig. 7c). Combining the results shown in Fig. 7b and c, we can see that the Pd@Pt with an OA/OAm ratio of 1 clearly exhibited higher ORR performance than the other two samples, with OA/OAm ratios of 2:1 and 1:2, respectively. Fig. 7d shows the cyclic voltammograms (CVs) of Pd nanosphere, Pt/C and various Pd@Pt nanoparticles in N2saturated 0.5 M H2SO4 solution, from these CV curves, we can see that Pd nanospheres show much larger peak areas than the Pt/C and various Pd@Pt nanoparticles for hydrogen adsorption/desorption, it may be ascribed to the high surface area of flower-like Pd nanoparticles, and the strong adsorption of Pd to the hydrogen. We can calculate the electrochemical active surface area (ECSA) for various Pd@Pt nanoparticles prepared with various templates composition. Furthermore, based on the assumption of monolayer dispersion of Pt, we obtained the Pt amount deposited on the surface of Pd nanospheres for each samples, and we assembled these results in Table 1. From Table 1, we can see the effects of OA and OAm additives on the performance of Pd@Pt coreeshell catalysts. With
the increase of the ratio of OA/OAm the ECSA and Pt loading are increased significantly, however, the catalyst with the OA/OAm ratio of 1/1 exhibits highest ORR performance (kinetic current). We suggest that the best performance of the catalyst may be result from its optimal morphology of the Pd nanospheres used as the core for the catalyst, and the OA/OAm ratio of 1 is an optimal ratio for the formation of desired morphology. Frankly, for the effect of OA/OAm ratio on the performance of final coreeshell catalysts, it needs further investigation. The durability of ORR electrocatalysts is one of the key issues for catalyst used for fuel cells. We therefore compared the electrochemical stabilities of these Pd@Pt core shell catalysts with the stability of commercial 20% Pt/C catalyst at room temperature and at high temperature of 70 C. At room temperature, the stabilities of both catalysts were compared by a chronoamperometry method, the Pd@Pt
Table 1 e Pt loading and electrochemical information from Pd@Pt nanoparticles based on Pd nanospheres synthesized using composite additives with various volume ratio of OA/OAm. Ratio of Electrochemical Pt loading/mg Kinetic current OA/OAm active specific at 0.6 area (ECSA)/cm2 V/mA vs. Ag/AgCl 0/1 1/2 1/1 2/1 Pt/C
1.22 2.25 5.12 5.04 1.22
0.92 1.98 2.83 3.80 5
0.23 0.85 2.89 1.22 1.19
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Fig. 8 e (a) Chronoamperometric tests results for 20% Pt/C catalyst and Pd@Pt nanospheres at 0.4 V for 30,000 s with a rotation rate of 900 rpm; the ORR polarization curves of Pd@Pt nanospheres (b) and 20% Pt/C (c) before and after the 10,000 cycles; column diagrams of the relative kinetic current at 0.6 V of Pd@Pt nanospheres and 20% Pt/C were inserted in (b) and (c).
catalyst presents better stability than that of 20% Pt/C catalyst. As shown in Fig. 8a, the current density loss at 0.4 V is 34% after 30,000 s for Pd@Pt catalyst, but 48% for commercial Pt/C catalyst. At high temperature of 70 C, the stability of the catalysts was investigated by a potential cycling from 0.4 V to 0.8 V (vs. Ag/AgCl) in 0.1 M HClO4 solution for 10,000 cycles. The both catalysts show almost same losses in onset potential. The Pd@Pt nanospheres show 45-mV degradation in half-wave potential after 10,000 cycles (Fig. 8b), while the degradation for 20% Pt/C is 55 mV (Fig. 8c). The Pd@Pt core shell catalyst shows the loss of 75.8% in kinetic current at 0.6 V (vs. Ag/AgCl) after 10,000 cycles (Fig. 8b). And the 20% Pt/C catalyst shows a loss of 83.2% of the kinetic current after 10,000 cycles (Fig. 8c). All the test results at room temperature and high temperature confirm the stability of Pd@Pt catalyst is a little better. Actually there is no big difference between them.
solvothermal method and then covered with a Pt monolayer using an underpotential deposition method. The coreeshell nanospheres exhibited substantially enhanced ORR activity compared to 20% Pt/C, and their Pt mass activity was 3.3 times higher than that of 20% Pt/C. It is suggested that this significant performance enhancement may be attributable to the nanoparticles' 3D dendritic morphology and the interactions between the Pt in the shell and the Pd in the core.
Acknowledgments This work is supported by the National Foundation of China (NFC Project NO. 21276098), the Nature Scientific Foundation of Guangdong Province, China (NSFGC Project NO. S2012020011061), and the Science and Technology 863 Foundation (STF Project NO. 2012AA053402).
Conclusion Appendix A. Supplementary data Pd@Pt nanoparticles with 3D nanospherical morphology and dendritic surface nanostructure have been prepared by a twostep approach. 3D Pd nanospheres were synthesized using a
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.07.025.
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